This update patches the "UnityPlayer" library in both the macOS and Windows versions of Widget Satchel to address a known (but to-date unexploited) security issue. It has been recommended that all games made with Unity Engine issue this patch, but the current practical risk to users is minimal.
Microsoft Defender (on Windows) has already implemented additional protections against this exploit for games that haven't been patched. Valve has said it will also issue additional protections within the Steam client (on all platforms).
As this update does not update any game code, the version number for this update is 1.0.3.1, and the version shown on the main title screen will remain 1.0.3.