A hash value is used to assign a unique numeric value to the contents of a file. Hashing usually involves converting a string of characters into a shorter, numeric string. The crucial point in hashing is that there must not be two encrypted messages that can be traced back to the same hash value. In this case, one speaks of a collision.
Hash values are interesting because no conclusions can be drawn about the message content from the hash value. The length of the hash value depends on the encryption algorithm and is 160 bits for the SHA algorithm and 128 bits for the MD5 algorithm.