It’s all specified on the jam page, but it’s really just about picking an older era and sticking to that as well as you can. If you want to go with 8-bit, go for an aesthetic that fits an 8-bit console, if you want 16-bit, same thing. If you’re going with arcade, just try to be as genuine to the era you’re aiming for.
Overall the idea is to just make a game that seems like it could have come out 20-30 years ago and challenging yourself to make it feel authentic.