Hello, and welcome!  Glad to have you join us!  Typically, the idea is to start working on the game after the jam starts.  But you can work on practicing general skills before then.  Like for instance, say maybe you want to learn how to include dialog in a game, you can start practicing how to do that now, and then use what you've learned in the next game jam.  And then when the jam starts, you can start developing specifically for the jam.

In fact, I highly recommend using time between jams to build your "toolkit" and skill set.  That also goes for art, music, etc.  Like if you're an artist, you can be working on an archive of sprites, models, etc, that you can then later use in jams.  So just as we say you can use pre-existing assets (art and sound assets, etc) for game jams, you can use pre-existing knowledge and general coding tools (even those you've made yourself) as well.

Does that make sense?