Actually, it isn't a bone of any kind. It's a nerve. Your nerves are a lot like your body's Internet. Different parts of your body send e-mail back and forth to and from your brain.
Nerves carry the signals.
When nerves get squeezed, sometimes the e-mail gets all messed up. This drawing is based on a cut-away view of your right elbow. Imagine a slice of your arm at the elbow, looking down toward your hand. Give yourself a funny bone examination. Use the first finger of your right hand to find the groove the ulnar nerve sits in.