More details concerning rules etc will be added to the description at a later point in time.
Last years GMTK jam had this in it's FAQ:
Does everything have to be made in the mentioned time period or can graphics, code, sounds etc. that were already created before also be used?
We request that you please abide by an honor code and do not submit a game or game content you were working on ahead of the jam as it isn't fair to the other jammers working from scratch. This rule does not include fair-use content from non-team sources, nor does it include any third-party or custom-built engines that you wish to use in the jam.