What is a cryptographic hash function?

Cryptographic hash

A hash value is a valuable tool used to determine if data has changed.

Often used in public-key cryptography, a hash is a mathematical algorithm that takes any piece of data and creates a fixed-size output called a hash.

As shown in the screenshot above, any change in the input data will result in a completely different hash value.
  
An example of using a hash: After signing a contract, create a hash of the contract. The hash is stored to use later.
To determine if the contract's integrity is intact the document, hash the document again. The resulting hash is compared to the stored hash.

If the two hashes match, you can say with mathematical certainty there is no tampering.