1: Jumping. Two 1s will let you jump higher, and a third one even more so.
2: Moving to the right. Each 2 will make you move faster to the right and slower to the left. Three of them will disable moving left entirely.
3: Screen borders. A single 3 on its own causes you to wrap around the screen in either direction.
4: Not much, but you can type 804 to immediately skip to the end of the game. Other codes with 4 don’t seem to lock in, so you can use this to keep typing more things without having to go to a checkpoint.
5: Spike immunity. Three of them will protect you from spikes entirely, while one of them followed by a direction (2 or 7) will protect you while you’re facing in that direction.
6: Slow fall. Having one of these in your tune reduces your jump to one pixel, but makes you fall very slowly.
7: Same as 2, but for moving left.
8: “Not”. Put this before a 1 and you can’t jump; before a 0 and you can’t fall; before a 3 and you can’t leave the screen.
9: ???
0: Down? Interacts with 8, but doesn’t seem to do much on its own.

It turns out the instructions are a bit misleading about what you should or shouldn’t do to successfully complete a room. Once you catch onto that, the real puzzle is figuring out how to complete all 8.