[StephenM wrote a good answer to that:](http://bitcoin.stackexchange.com/questions/35699/what-is-a-block/35700#35700)

>5. The difficulty (or nBits as it is called in the code) is shorthand for how difficult it is to solve the block. Essentially, it encodes a target value, and the hash of the block (when treated as a 256 bit integer) must be below that target value to be considered solved.