A hash function is a mathematical algorithm that transforms input data of any size into a fixed-size string of characters, typically a sequence of numbers and letters. This output, known as a hash value or digest, uniquely represents the original data, making it easy to compare and verify information without revealing the actual content. Hash functions are widely used in computer science, particularly in areas like data integrity, password storage, and digital signatures. They ensure that even a small change in the input will produce a significantly different hash value, enhancing security and reliability in systems like blockchain and cryptography.