A hash method takes data and outputs that data as a hash("unique encoded string")
Sort of like a fingerprint to that file or data
There is this great thing called HashMaps, that is similar to an array data structure but has a look up time that is constant. These are looked up by "keys", instead of a sequence of indices.
Hashing can be a useful tool for spotting any nefarious intervention with files.
One party can hash the file on their end, and while downloading on the other end you can compare the hash data to the original user to confirm that the file hasn't been manipulated in any way.