If you perform a jump right as you're directly next to or on top of an opponent, you'll jump on their head and leap up.This is what's known as a footstool. You're using your opponent's head as a stool for you to step on and jump off of.
It works differently on a grounded opponent than it does for one in the air. If you do it while grounded, your character will jump off the head of the opponent and they'll be locked into an animation for a small second. It'll look like this:
In the air, a footstool can be an effective gimping tool if timed right. You'll perform the same jump on their head, but since they're in the air as well, they'll go into a tumble animation for some time in which they can't perform any inputs to break the animation lock.
Best way to practice this is to go into training and just intuitively jump when you're right above the opponents head. It's pretty simple to get the hang of.
If you're interested in something lengthier, check out:
http://www.ssbwiki.com/Footstool_Jump
Why I didn't just link to that in the first place? Who knows, really.