Insert your SteamID, the site will ping your Steam profile every minute, keeping track of the last game/server that you were playing on, allowing you to rejoin if the connection is lost.
I enjoy playing Awesomenauts, but sadly it lacks a rejoin button. When for some reason I lose the connection there is no way to find the same match, now, this keeps track of it for me.
No, only with games that use Steam based servers. With supported games you will see a 'Join' button on your profile (and here) while playing.