Proton is a compatibility tool which allows running Windows only games from within a Linux desktop. The reason for releasing and supporting this type of tool is unclear. Linux use is less than 1% according to the Steam Hardware & Software Survey. That being said, it is always good to increase your marketability.
Proton was very well received within the Linux community. Under the hood, Proton integrates Wine, DXVK, and FNA in order to emulate the Windows APIs commonly used by many game engines. This compatibility is still very much in its infancy and users are encouraged to report any bugs through the Proton Issues on their GitHub.
This is a beta-only feature available by enabling the beta client. To do so, simply go to your Account Settings in Steam and enable to Beta participation.
The usage within the Steam client works pretty well. Games that are Windows only will be now be visible within the Steam Library. Within a games settings, specific Proton versions can be selected.
Compatibility for games varies. The Steam Issues tracker is a good place to look whether a particular game has any issues.
I was able to run Skyrim SE reasonably well out of the box. I did run into some issues with sound which required manual intervention. With the default FAudio libraries shipped with Proton 4.2 and prior, the NPC voices do not work. This is described in the The Elder Scrolls V: Skyrim Special Edition (489830) issue. The work around is to compile a more recent version of FAudio and override what currently resides in the Proton folder.
git clone git://github.com/FNA-XNA/FAudio.git cd FAudio; mkdir build; cd build cmake .. -DXNASONG=OFF -DFFMPEG=ON make -j4 cp libFAudio.so.0 ~/.local/share/Steam/SteamApps/common/Proton\ 4.2/dist/lib64/libFAudio.so.0
This needs to be done everytime Proton is updated since the custom library will be overwritten on update. With an updated FAudio library in place, I am able to hear the NPC voices and found no other problems running Skyrim.
This is an exciting time to be in the Linux ecosystem. More and more companies are coming together and supporting our beloved kernel. Since it is in its infancy, Proton will have some issues. But the Linux community is great at coming together and documenting and supporting open source initiatives like this.