does the theme includes: "120 second", "slam" and "transition"
Those are the themes, yes. You can use any or all of them; the words to their left are meant more for inspiration/optional guidance/flavor.
But the Jam starts in 21h, how can themes be already known?
not all jams reveal the theme(s) when it starts
My guess is this is a beginner-friendly jam :) So you can do a bit of planning before the jam starts!
Yep, precisely!
