It is the sum of the numbers 4 8 15 16 23 42. These numbers are used throughout the series. Here are a few examples...feel free to add more! These are Hugo "Hurley" Reyes' winning lottery numbers. When he was in a mental hospital (and Libby was as well), a fellow patient recites the numbers incessantly. In the Swan hatch, the numbers appear on the hatch door (we see them imprint them in Season 5) and DHARMA employees (then later Desmond and the Oceanic 815 survivors) enter these numbers into a computer every 108 minutes "to save the world," by preventing a build up of electromagnetic energy. You notice the flight number contains two of the numbers. In Season 6, we learn Jacob assigned one of these numbers to six of the Oceanic 815 survivors: 4 = Locke; 8 = Reyes; 15 = Ford; 16 = Jarrah; 23 = Shepherd; 42 = Kwon (but the Locke-ness Monster says he doesn't know if it's Sun or Jin). Jacob's ghost asks Hurley to go with Jack to the Lighthouse and turn it to 108 degrees "to help someone get to the island." Jack notices that there are names next to degrees, namely 23--Sheppard. When he turns it to 23 degrees, he sees his childhood home reflected in the glass. Therefore 108 degrees may correspond to a person. My theory right now is that it might be Desmond...Eloise Hawking said the island wasn't done with him, however, the last time we saw him (apart from in Sideways-flash world) he was recovering from a gunshot wound from Ben Linus. However, 108 degrees may not actually correspond to anyone because it is later revealed that Jacob wanted Jack to "know how important he is," meaning his mission for Hurley was a smokescreen of sorts, in that the true purpose was hidden from both Jack and Hurley. This may be one of the ways that Smokey meant Jacob was "manipulative."