A hash function is a cryptographic algorithm that converts input data into a fixed-size string of characters, which appears random. In cybersecurity, hash functions are widely used for data integrity and password storage. When passwords are hashed, they aren't stored in their original form, enhancing security; the system stores only the hash. During authentication, the entered password is hashed and compared to the stored hash. If they match, access is granted, protecting sensitive information from unauthorized users.