Its exactly as it sounds. A big black rock/cliff about 300m out into the ocean. You can try negotiating the peninsula it is attached to, but it isn't recommended. Be prepared for a swim. Once you reach the rock, it is easy enough to climb up and leap off as many times as you please.
The Road to Hana is as beautiful as it is dangerous. It is a stretch of thin highway (64 miles) that spans the NE coast of the island. There are tons of unbelievable scenic spots along the way *including a few from Jurassic Park and Lost. If you want to test your nerves and driving skills, please be careful.