Hashes are functions that take some arbitrary input and return a fixed-length value. The particular value depends on the given hash algorithm in use, such as SHA-1 (used by Git), SHA-256, or BLAKE2, but a given hash algorithm always returns the same value for a given input. Have a look at the full list of hash functions for more.
As an example, the input:
Hello world would be represented by SHA-1 as: