A checksum tells you whether your version of a file has been modified or not.

A checksum is the result of running an algorithm, called a cryptographic hash function, on a piece of data, usually a single file.

By comparing the checksum you generate based on your version of the file with the checksum provided by the source of the file, you can be sure that your copy of the file is authentic and error-free.

A checksum is sometimes also called a hash sum and less commonly a hash value, hash code, or simply a hash.