A hash function in cryptography refers to a mathematical function that converts a numerical input value into another compressed numerical value. The input to the hash function is of arbitrary length but the output is always of fixed length.
A hash function is also known as a hashing algorithm or message digest function.
To put it simply, a hash function takes a group of characters and maps it to a value of certain length.
That is to say, for any x input value, you will always get the same y output value whenever the hash function runs.
f(x) = y
This means every input has a predetermined output. The hash value represents the original string of characters but it is usually smaller than the original. The value you get after processing a set of data through a hash function is called a hash value or message digest.
Note that the input can be any data like numbers, files and other types of files. The process of transforming a given set of data to a specific hash value is called hashing the data. The hash value or message digest is always in form of a hexadecimal number.
Since the resulting value after hashing is much smaller compared to the data that passed through the function, makes hash functions to act like compression functions.
Tools related to Hash Function: Encrypt and Decrypt Online