The standard rules of MahJong apply, match like tiles or in this case bubbles, if they are free. Free bubbles are not blocked on the left, right or covered by another bubble.
Points are earned for matching bubbles. To continue to the next round you must earn enough points to qualify.
If there are no matching bubbles left and you've earned enough points to continue to the next round then use the swipe gesture to advance to the next round.
Earn bonus points and increase the score meter by making 5 consecutive matches without any mistakes.