In Episode 221 of Naruto: Shippūden, what was it that Naruto signed about? When the Great Toad Sage reverse summoned Naruto, he signed something about a key and I don’t understand what just happened. 

What did he do? Did he keep back the Nine Tails or what? No spoilers please.
That scroll was not a contract but rather, the key to the Fourth Hokage's seal.

In Chapter 489, Gamatora reveals it after Naruto's inquiry. To quote,


  The key that locks and unlocks the Fourth Hokage's Sealing Jutsu that's on yer belly! With this, ya could even let Nine Tails out to run free. 


Naruto accepting it means he now has control of the Fourth Hokage's Sealing Jutsu via the key.