A checksum is a value used to verify the integrity of a file or a data transfer. In other words, it is a sum that checks the validity of data. Checksums are typically used to compare two sets of data to make sure they are the same. Some common applications include verifying a disk image or checking the integrity of a downloaded file. If the checksums don't match those of the original files, the data may have been altered or corrupted.
Below are some examples of tools and ways to verify checksums in common Operating Systems (OS).
Open Powershell and type: "get-filehash filename.xxx"
Windows Graphical: Download and run a hashing program such as HashCalc: https://www.slavasoft.com/hashcalc/
Open the command line "terminal" and type "shasum -a 256 filename.xxx" https://www.dyclassroom.com/howto-mac/how-to-verify-checksum-on-a-mac-md5-sha1-sha256-etc
Open the command line "terminal" and type "sha256sum filename.xxx"