Beginner
Cryptography
Hash function
A one-way function that turns any input into a fixed-size fingerprint.
In simple terms
Think of a hash as a fingerprint for data. The same input always gives the same fingerprint, and you cannot work backwards from the fingerprint to the input.
Analogy
A blender: fruit goes in, a smoothie comes out — and you can never turn the smoothie back into the original fruit.
Going deeper
Formally, a cryptographic hash must be preimage-resistant (you cannot find an input for a given digest), second-preimage-resistant (given one input you cannot find another with the digest) and collision-resistant (finding any two inputs with the same digest is infeasible).
Common mistake
Hashing is not encryption. There is no key and no way to reverse it.