Above is a screenshot in-engine of Nightborn's Demo with numbered labels keyed to the explainers below.
Nightborn is an RPG, with deep lore, various combat, puzzle, dialogue, stealth, and magic systems. The in-game interface is a utility to communicate these features and current states with the intent to inform the player of their options.
1. Location Types
In the world of Nightborn, there are different location types which inform the player what to expect from the encounters in that area.
A Sanctuary is a location where combat is forbidden by magic or strict social graces. There are areas where Supernatural beings have agreed to treat as neutral ground for their dealings. Goblin Markets of the Fae, Underworld Nightclubs, and Nightborn Courts are some of the places you may fund under a Sanctuary spell. 
Public Areas are where you will encounter most mortals. Nightborn are generally expected to behave themselves here, but a little violence is forgivable, especially if one cleans up after themselves. Too much naughtiness will bring the ire of the Mortal Authorities, Vampire Hunters, or a Nightborn Enforcers depending on the severity. Keeping a low profile and just doing your business is expected of Vampires if they way to maintain good standing in Nightborn Society. Most of the Hubs in the main Nightborn game are Public Areas. 
Combat Zones are locations which are occupied by already hostile entities. Most beings encountered in a Combat Zone will attack your character on sight. Surviving a Combat Zone requires being well armed, slick and stealthy, and/or supernaturally empowered. Killing in a Combat Zone does not have any repercussions with Hunter attention, as the Hunters either hunt what occupies the zone or the zone is occupied by Hunters. There may be benefits to being undetected in these areas, however. 
Holy Ground is the opposite of a Sanctuary. This location is actively hostile to Nightborn. Vampires are weakened when in this location and there may be additional further harmful effects unique to each Holy Ground. Hunters like to make bases of operation on Holy Ground for this reason. It is harder to remain stealthy or in disguise on Holy Ground, but not impossible.
2. Stat Bars
There are three Stat Bars.
The top blue bar displays Health. Vampires have an innate slow regeneration. Health is restored upon feeding. Faster Health Regeneration, an increased Health Bar, Damage Reduction, and Fast Healing are all effects from various Nightborn Legacy Powers.
The middle red bar display Blood. Blood is what fuels a Nightborn's Legacy Powers. Blood is restored upon feeding. Blood regeneration, an increased Blood Bar, Blood Cost Reductions, and Blood Bar Restoration are all effect from various Nightborn Legacy Powers.
The bottom purple bar displays Passion. Passion starts a zero and ramps up from various stimuli such as getting hit, being seduced (wanted or otherwise), or failing at a puzzle. Passion represents the character's Temper, Ambition, Fear, Desire, and other strong emotions. When this bar hits its threshold the character may go berserk and the player temporarily loses control. Vampires are deeply emotional beings who feel much stronger than mortals, and this becomes hard to contain. There are ways to spec a character to dampen Passion gain to achieve a cold and unyielding heart. There are also ways to spec into Passion to use it as a resource to enhance Powers and optionally lean into the boosted Berserk state.
3. Equipped Weapons
While Nightborn characters do have an inventory, they may only carry on melee weapon and one ranged weapon at a time. Vendors will happily sell your character weapons and ammo. The keyboard number buttons toggle which weapon is equipped. 1 toggles unarmed, 2 toggles melee, and 3 toggles ranged. Left clicking the mouse initiates an attack.
The basic unarmed state with no weapon equipped. Nightborn are quite strong, so combat with fists are no joke. Monstrous claws and other Metamorphosis forms also count as unarmed combat.
Melee weapons such as a simple knife will do additional damage in combat. Other melee weapons include baseball bats, axes, and swords. Vampires are much stronger than mortals, so non-enchanted weapons may break under the stress of their impacts.
Ranged weapons require ammo. The top numbers show how much ammo is loaded / the max ammo number that can be loaded. Under that, it shows the ammo number in the inventory which is not loaded in the weapon. While ammo can be found on enemies using that type of ammo, ammo is not often found in the world as loot. The R key reloads a ranged weapon.
4. Legacy Powers
The Nightborn Demo will launch with only the Legacy Power under the Nightborn Entitlement of Carnality. The Carnality Legacies are Brutality, Glamour, and Blood Magic. All Entitlements have three Legacies, one of which is a Magic Legacy. All Vampire Powers cost Blood to use. Magic Powers tend to be more powerful than other Legacy Powers, but require Blood and additionally consume a spell component from you inventory, kind of like a Magical Ammo. The Mouse Scroll switches between powers that have been purchased via experience points. Right-clicking the mouse initiates the active Legacy Power. 
Brawn is the starting Legacy Power in the Brutality tree. Activating Brawn imbues the player character with physical might, making their muscles bulge and their expression fierce. While in this state, their dialogue Intimidation is increased and their unarmed and melee strikes are greatly enhanced. 
Charm is the starting Legacy Power in the Glamour tree. Charming in dialogue makes the target treat the player character like a friend, opening conversation possibilities. When used outside of dialogue, Charm makes the target stop what they are doing to stare at the player character for a few second. The more powerful the target, the less Charm effects them. 
Syphon is the starting Legacy Power in the Blood Magic tree. The Syphon spell requires a Crimson Candle, made from virgin blood, in order to cast. Spell components may be found in the domains of those that traffic in magic, including being sold in Magic Shops. The number of the corresponding components will be displayed next to the component's symbol. Without a component, the spell may not be cast. Syphon will drain all the blood out of a target at a range. This is an instant kill when used against a mortal and will damage and temporarily disable the powers of Supernatural targets. Syphon may be used to extract information based on context and clever application. Nightborn Magic is often more versatile than expected.