This is the native client (i.e. binary) for the Youtube Play With MPV Chrome Extension. You need to install this in order for the Chrome extension to be able to open a Youtube video in mpv.
If you install the Native Client after you install the Chrome extension, make sure to disable, then re-enable the Chrome extension.
Windows 64bit
Download the installer here: https://github.com/Darkle/youtube-play-with-mpv-native-client/releases
The installer includes the mpv and youtube-dl binaries so you don't have to install them yourself.
Note: if you would like to update either the mpv binary or the youtube-dl binary after installation, you can manually replace the files. They are located in C:\Users\Username\AppData\Local\Programs\ytplaywithmpv.