Proton is a Steam tool that enables Linux users to play most games otherwise only playable under Windows.
To enable Proton:
To verify the compatibility of a given Windows game with your system, visit ProtonDB and search for the game by name. Reading the comments, you will find that some games work better (and sometime only) with an older version of the Proton library. To get around this issue, Steam allows you to select a different version of Proton from the game's Properties panel.