Hey! The idea behind Jamuary is to get something done every day. It's usually a new piece of music every day, and it can range from a musical idea to a full track, it's really up to the jammer. It's also totally normal to skip some days (I never managed to get a perfect streak)
For this specific jam, the first 23 days are really about generating a lot of music: many small, daily jams indeed. Near the end, it makes sense to pick what you have already done and finalize the tracks, unless you want to keep generating.
Keep in mind that you don't have to submit 31 songs. You may even decide that you just want to submit one song, so no pressure on working every day and having 31 high-quality, mixed and mastered tracks.