File checksums are mathematical values that are calculated from the data of a file and are used to check the integrity of a file. This means that the checksum can be used to check whether the file has remained unchanged since the checksum was calculated.
Most checksum algorithms use complex mathematics to calculate a unique value from the data in a file. If even a single bit in the file changes, a completely different checksum value is calculated. This means that a checksum check can be used to ensure that a file has not been changed while it has been stored on an external storage medium or transferred over a network, for example.
An example of a checksum algorithm is MD5. This is one of the early algorithms of its kind and is often used to check software downloads and other digital data. However, there are also modern algorithms such as SHA-1 or SHA-256, which provide greater security and better integrity checks.
Here you can calculate the checksums or hash values (MD5, SHA-1 and SHA-256) for a file.