Thank you for playing it, I am glad it is fun to play. :)
Drops do the following (copying a part of my comment on the game page):
White/Red: Heals 1HP up to 5HP.
White/Blue: Heals 1HP up to 9HP.
Red/Yellow : Weapon power-up (double damage and drop chance).
If picking up a drop would do nothing, then it is not picked up, similar to how in many games drops for maxed-out ammo and health are ignored.
Also, you can start from any level by pressing keys for up/down or left/right. Upon losing, it starts with the last level played. There is a total of 25 different levels (which then just loop indefinitely like in e.g. many NES games).
I did not have time to write a proper description of the game because of the jam submission deadline (I had less than 2 minutes left). Also, this is my first game jam and I did it rather hastily and I am still not completely sure how does all of that work (e.g. I am not sure if it is okay to edit the description after the deadline, so I will just leave it brief as-is, and add additional info in comments).