It is an arbitrary number. The network began at difficulty 1 which requires on average 2^32 attempted hashes to solve a block.
Difficulty 11 million is simply 11 million times more difficult than difficulty 1 (i.e. currently it takes 2^32 * 11,187,257 hashes on average to solve a block).
Thank you, makes sense! Why did it take 2^32 attempts with difficulty 1? I know Bitcoin works with a target hash and SHA-256. There must be a (complete) way to calculate the average number of hashes required with a certain target. Anyone can help me with this?