Proton is a tool for use with the Steam client which allows games which are exclusive to Windows to run on Linux and macOS operating systems. It uses Wine to facilitate this.
Steampowered forum post:
Most users will prefer to use Proton provided by the Steam client itself. The source code is provided to enable advanced users the ability to alter Proton. For example, some users may wish to use a different version of Wine with a particular title.