You can only remove groups of two or more tiles that are connected horizontally or vertically. When you remove a group, any tiles above them fall down to fill in the gaps. When you clear out an entire vertical column, the other columns shuffle up to fill in the gap. The more tiles you remove in one tap, the more you score. In general, removing N tiles in one tap scores (N-2) x (N-2) points.
When there are no more groups of two or more tiles left, the game is over. Clear as many tiles as possible for a huge bonus. If you have cleared all tiles, you may advance to a new round.