You will get rewards for collecting lanterns  at mahjong levels and completing tasks from the lantern bar. You need to catch a latern (match a tile with one) in 5 moves, otherwise it will disappear. Lanterns turn up only at new levels.

 If you complete levels at the first attempt, the number of lanterns doubles. The lanterns you collect will be added to the lantern bar. As soon as you complete the bar, you will get a reward from a dwarf. Collect all lanterns, and you will receive the grand prize! 🎁