It's middleware for game launchers like Lutris, but specifically for Wine games. It uses GloriousEggroll's custom Proton builds instead of stand-alone Wine/DXVK/etc.

Note that GloriousEggroll more or less discontinued his custom Wine builds, and has been encouraging people to use his Proton builds via this tool instead. It might make things easier or harder, depending on your needs and workflow. I expect it will be easier for casual Linux users when combined with a GUI.