Checkpoint Challenge Tokens are a new currency that are used specifically to enter the Checkpoint Challenge event.  It costs 1 Checkpoint Challenge Token to enter the Checkpoint Challenge, or to re-try the challenge if you are knocked out of the event.

Checkpoint Challenge Tokens can be earned as rewards in Checkpoint Challenge chests, through daily ad watches and acquired through Gem and in-app purchases.
Checkpoint Tokens can only be acquired with Gems, purchase and ads when your balance is 0.

Please note that only 20 Checkpoint Challenge Tokens can be stored at one time.