During play, you'll sometimes be awarded TOKENS.

Tokens can be used to unlock new puzzles. Once a puzzle is unlocked, it remains unlocked until it has been completed. You can access puzzles unlocked on previous days through the Issues Overview screen.

Tokens can also be used to replay completed puzzles.

You can see how many tokens you currently have in the status bar area at the top of the screen. You can tap the token meter to go to the in-game Store where you can buy or earn additional tokens.