A hash value is a fixed-size string of characters generated by a hash function from input data of any size. It serves as a unique identifier for that data, allowing for quick comparisons and data integrity checks. If even a small change is made to the input, the hash value will change significantly, making it useful for detecting alterations. Hash values are commonly used in various applications, including data storage, digital signatures, and password management. They help ensure the security of information by allowing systems to verify that data has not been tampered with, thus maintaining the integrity of files and communications.