You need Hearts ❤️ to play different levels.
You can have up to 5 Hearts. (8 hearts if you have the TANY Pass 🎟️)
You can see how make Hearts you have left on the upper-left corner ↖ of the screen.

You will lose a heart if you fail to clear the level.
You will get a new Heart after 30 minutes 🕓.

You can also exchange Gold for Hearts, or ask a friend or teammate 🤗 for one.