A day after the finale of one of the most ambitious, confusing and fascinating shows on television, many fans are still confused. The general gripe is that the mystery of the island hasn’t been clarified. I’ll try my best.
The best theory I can think of: MIB(Man in Black) and Jacob are manifestations of the good and bad in the characters. When the characters landed on the island they were broken people (emotionally tormented, angry, murderers, hopeless, etc), and the island gave them a chance to redeem themselves. MIB represented their “old” self or the bad in them that was threatening to leave with them while Jacob represented the good that was trying to redeem them and prevent them from returning to their “old” destructive ways (hence: he was trying to stop MIB(the bad in them) from corrupting them fully).
Here are some examples: Kate’s name was crossed off the list of guardians because she became a mother to Aaron, which changed her life for the better (redeemed her). While Claire, having lost Aaron, became lost and self-destructive herself, hence falling under the influence of MIB, as did Sayid, who later redeemed himself by sacrificing himself with the bomb.
Scientifically speaking: the island has a strong electromagnetic core that disrupts poles and shifts time, hence the time travel, polar bear in the tropics and smoke monster. Spiritually speaking: the island is a ‘cleansing’ place that redeems people through obstacles.
Once people left the ‘bad’ behind, someone was needed to keep it on the island or else it would leave, hence the island needed a guardian (to keep the evil on the island). But that guardian had to be the person in-need of the most redemption who hadn’t gotten it. Hence: Jack (who was still tormented), Hurley (who had no purpose in life) and Ben (who committed the most crimes but was still alive, unlike Sayid).
I hope this clarified things a bit.