The ground jump lets you perform a straight vertical jump, allowing you to grab ledges that you couldn't have grabbed before. It can be done as Link and Zora Link.
There are three different ways to store a ground jump:
After you've stored it, just backflip to ground jump. This video demonstrates all three methods in order:
No, it has been obsoleted by the gainer in Majora's Mask. If you're having trouble with a gainer, though, you can almost always ground jump, and many failed superslides ultimately result in a ground jump, so the trick is good to know.