There are sleeping drivers in some cars. You can spot these drivers by the ‘ZzZ’ bubbles above their cars. A sleeping driver needs to wake up before their car can move. To wake up a driver you need to make a match on any adjacent tiles.

You might also see a Sleep Sign on some levels, when a car drives over it, its driver suddenly gets very sleepy and the car stops and adopts the 'ZzZ' sleeping bubble. You need to make a match adjacent to that car to wake up the driver and let them move on.