mpvshots is an automatic screenshot uploader for mpv. After a bit of configuration, you'll be able to automatically upload all screenshots you take in mpv to Twitter.
There are two major components of mpvshots. First, there is the eponymous command line tool, mpvshots, which is your primary interface to all functionality. The second component is the daemon which does all the actual work, which is named mpvshotsd. You do not have to manually control this daemon, and should use mpvshots to manage it.
To upload a screenshot to Twitter (or somewhere else, as different backends are theoretically possible), several things need to happen.
First of all, obviously, does the screenshot need to be taken, and mpvshotsd must be somehow notified of this action. To simplify things, we simply have mpvshots iniate the screenshot via mpv's IPC, then hand the screenshot off to mpvshotsd via another IPC call, which will queue it for upload. You can in fact replace mpv's screenshot command by a call to mpvshots, which makes the entire process transparent, you just keep taking screenshots as usual.
To work, mpvshotsd obviously needs to be able to access your Twitter account. To do this, mpvshots does OAUTH and saves the credentials for the daemon to access. It will save them in ~/.mpvshots/. This is also where mpvshots' configuration file will eventually be found.