To "step on" something means to walk or run on top of it:

I think I stepped on a piece of gum.

Note that we only "step on" things with our feet. In a car, you "run over" something:

I ran over something that fell out of the truck in front of me and it popped my tire.

With your body, you "roll over" something:

Some parents don't want their babies to sleep with them because they're afraid of rolling over them.