Basically, the tiles do not replenish, and once you have made all possible moves the board locks and the next level begins, in which the board is filled again. The ones that remain are locked and can not be moved, but can still be used in a combination, or un-locked by geting a combo next to them. If the screen fills up with locked pieces and you have no moves, you lose. Your objective is to beat it.
This can be done in two ways:
Play through 50 levels:
Complete all 50 levels. Look for big combos;getting a star (surrounding a tile with tiles of the same colour) or linking three multiplier tiles of the same color together destroys tiles around them, so use this to your advantage. Obviously, geting the locked tiles out is essential, to prevent your board filling up. Remember, if you need a break and quit, it will save your progress.
Empty the board:
If you should use up every tile on the board, you win. Easy as pie. It is suggested that this is the easiest way to accomplish this achievment, because in the initial few levels the color variations are rather limited, and by linking together a few stars you can easily destroy all the tiles.