A game jam is a competition over a short period of time for someone (or a group of people) to make a game from scratch with the same theme as everyone else (As given by the competition judge), This is very similar but instead of making a game from scratch you make a minecraft map from scratch and upload on planet minecraft with the title of the map followed by (MAP JAM 1) and your map will be voted on by everyone else and the winner gets a special prize.

If you want to participate then join the discord channel here: discord.gg/Ya6DRF
Then you will be able to create teams and share ideas with others, the theme will be announced on the discord and you will have from Saturday 5PM GMT till Sunday 5PM GMT meaning you will have 24 hours to make the map and upload it to planet minecraft.

Rules:
The Map must be uploaded by the deadline or it will be disqualified.
The Map must contain (MAP JAM 1) in the title. (preferably at the end)
The Map must be appropriate for all ages. (remember others have to play the map too)
The Map has to be public.
You have to have joined the discord server and put your discord name in the description of the map.
Custom texture packs are allowed but mods and data packs are not.
And lastly have fun!