Definition
A hash function maps data of any size to a short value. Cryptographic hashes such as SHA-256 are one-way and collision-resistant: you cannot recover the input from the hash, and you cannot easily find two inputs that share one. Comparing hashes is how you check that two copies of a file, or a value and its earlier state, are the same.
Relevance
A hash is how you compare data without comparing every byte yourself. Two files with the same SHA-256 hash are the same file for practical purposes. One changed byte produces a different hash, and the hash does not reveal the original data.
Example
You publish a content file and its SHA-256 hash. After a download, the game hashes the file it received. The hashes match, so the file arrived intact. A single flipped bit in transit would have produced a different hash.
Connected
Synonyms
Checksum, Digest