From the Character Creation section:
You are a member of the Frog Clan.
The clan has a duty to keep the Celestials imprisoned lest they make good on their ancient promise to destroy the Earth.
You, personally, have a duty to imprison a specific Celestial. This duty has been handed down from Frog to Frog over the eons. When one Frog dies there is always another prepared to be the Celestial's jailor.
You have given up a piece of yourself in order to hold your Celestial in place. But it works both ways; you get to use a bit of that Celestial, too.