As the difficulty increases, the number of possible valid hashes decreases. With less possible valid hashes, it takes more processing power to find a valid hash.
The nonce iterates until the hash is valid. In our case, a valid hash has at least four leading 0’s. The process of finding a nonce that corresponds to a valid hash is mining.
Someone once said that naming things is one of the hardest jobs in computer science. I couldn’t agree more. But structuring folders and organizing files is a close second.