Jam started hours ago, but as of 6/30/2018 11:14, no theme has been announced. Dead Jam?
a bit late for this, but a theme is just a word or concept you can adhere to for your game. Mainly just to give you direction but in most jams it isn't too strict. For example the theme "keys" could feature a game mechanic where you unlock doors with keys or "waves" could just have you game set next to a beach.