Macbeth is referring to Malcolm as a potential obstacle to his own path to the throne. He sees Malcolm as a "step" that he must either "fall down" (eliminate) or "o'er-leap" (surpass or overcome) in order to achieve his goal.
Macbeth might choose to do several things:

Eliminate Malcolm
Plot against Malcolm
Try to surpass Malcolm

Can you help me in explaining each of these potential actions Macbeth will do?