In Merge Dragon you need Chalices to play levels on the World Map. In the image above, the arrow points to where you can find your Chalices in Merge Dragons. If you are new to the game you will have 14 Chalices, and eventually you will have a max of 7 Chalices.
The Chalices take an hour to refill, you can also watch a few ads a day to get extra Chalices. You can also spend gems to refill your Chalices but I do not recommend it.
In the image below you can see where the # of Chalices required to play a level is indicated.