You may work on general purpose code before the jam begins, but not game code or assets.
"General Purpose" code refers to code that should function properly and identically when plugged into (almost) any game.
For example, I have code I wrote for one engine that allows me to easily play sound effects in my games. I copy-paste this code, verbatim, in pretty much every game I make in this engine, regardless of the type of game it is. That's general purpose code.