Original: Most cryptographic hash functions are designed to take a string of any length as input and produce a fixed-length hash value.
Simplified: A hash function takes a string of any length as input and produces a fixed length string which acts as a kind of "signature" for the data provided. In this way, a person knowing the "hash value" is unable to know the original message, but only the person who knows the original message can prove the "hash value" is created from that message.