According to Lostpedia, John Locke was one of the middle section survivors of Oceanic Flight 815. When he boarded the flight he was paralyzed, but his paralysis was healed upon crashing on the Island. He and Jack Shephard were philosophical opposites and used to share leadership among the survivors. If you don't mind spoilers about the later Lost storyline, you can learn more at Lostpedia:John Locke.
You might also want to learn more about the real-world philosopher John Locke -- the naming wasn't a coincidence!
- For additional people/things named "Locke", see Wikipedia:Locke.