A checksum file is a text file that lists cryptographic hash values for one or more downloads. After you fetch a package, installer, or release archive, you can compute its hash locally and compare it with the value in the checksum file. If the numbers match, the file has almost certainly arrived intact and unchanged.
In cyber security, checksum files help users and automation detect corruption, accidental transfer errors, and some tampering attempts. They are common alongside software releases, mirrors, and update systems, especially for Linux packages and tarballs. Defenders use them as a fast integrity check, but they are strongest when paired with a trusted signature on the checksum file itself. Attackers may try to replace both the download and the checksum file, which is why signed release artifacts and verified signing keys matter.