A cryptographic hash is a mathematical function that takes a file and produces a relative shortcode that can be used to identify that file. A hash has a couple of key properties that make it unique. Only a particular file can produce a particular hash, and two different files will never produce the same hash. It cannot be reversed. You cannot work out what a file was by looking at its hash. Hashing is used to prove that a set of data has not been tampered with. It is what makes Bitcoin mining possible.