The rules say that pre-existing assets (such as music, sound effects, and graphical resources) may be used, but any new content made for the game must be created during the jam period. Does this mean I am allowed to make art or music for the game only during the jam period or can I do that before it starts?

If you are making assets SPECIFICALLY FOR THE GAME then those must be created during the jam period. If you just want to use existing assets, like art you made for another game or a paid tileset, in your submission, then that is acceptable. Anything specific to your entry (besides concept art or brainstorming) must be done during the jam period.