the tv show lost,,,the heart of the island?

[g man m]

need help with the last episode,,,, that rock (the heart of the island) that desmond pull out,, does that mean that the island is gonna fall down (sink or whatever) ok so why does jack go back and put the rock back in? is it so stop the island shaking? if so then why? or was it so save the others that went on the plain,,, he should of just not bothered with that stone and just got on the plain with the others,,,,, i dont understand