Your first move drops a blob one dot short of popping on any empty square.
After that you can only tap your own blobs to add a dot.
When it reaches the pop count (4 dots by default, 2–8 in the lobby) a blob bursts, sending one dot to each neighbour (two at 8) and taking them over; any extra dots stay behind. Bursts chain.
Lose all your blobs, or run out of time with the clock on, and you're out. Last color standing wins.