A number or string of text that is calculated from data when data is sent from one place to another. When the data and checksum arrives at its destination, the data is checked to verify the checksum matches the one provided. If it is a match, the data is believed to be error free and not compromised in any way.
Verifying the file checksum helps you make certain the file was not corrupted during download, and that it is not infected with malware or other malicious software. The checksum is typically represented as a long string of alphanumeric characters. For example, an SHA-256 checksum may look like: