The Forest Temple was an on-island location dedicated to the god of the island: John Locke, who felt time-traveling and setting up his own prophecy would make for some funny lols. After he was killed, The Man In Black was so mad that he never got a temple (even though he asked his Mother for one). So one day he called "givesies" and destroyed everyone inside.
Inside the Temple, it is said that the Forest Pendent awaits the one who is brave enough to brave the dangers within. The pendent is said to unlock all the mysteries of the Island, killed the Smoke Monster, and grant one wish to the finder. Also, it looks a lot like a baseball. If you see it, NEVER let it it out of your sights.