It depends on your game, but generally you should have a clear and simple goal (push the red button, reach the flag, defeat the boss (this one is banned from this jam), collect the chaos emeralds) that you give the player from the very beginning.

When looking at your goal a player should be able to instantly tell that this is something that they have to touch/interact/destroy. Bright contrasting colors and animations as well as unique sprites should get this point across.

Also don't get carried away and make too many levels, just enough to get your point across without repeating yourself and boring the player. There will be many games to rate and the raters don't have too much time (also I suck at platformers and am very happy if I get to finish one), plus using your time for polish is a much better investment.