Difficulty refers to the level of computation work required to verify a block (via calculating the nonce) in mining. Specifically, it is an indicator of when the hash value of a block is less than a given target value. The higher the value is, the more difficult it will be to mine a block. Combined with hashrate, difficulty is used in calculating the indication of the number of blocks generated.
Therefore, although the block generation rate is said to be once every 10 minutes, it is not strictly 10 minutes, but is adjusted so as to be as close to 10 minutes as possible.