Checksum is a system by which a computer can verify the contents of a file against a known record, ensuring that the contents are correct and undamaged. A bad checksum error results when the file fails this check, meaning that some part of the data no longer corresponds to the original. If your data fails a checksum verification, it usually means that you need to replace it from a backup or otherwise reconstruct the affected file.
Function
If, for example, you have a list of numbers you need to transmit to a coworker, and need to make sure he writes down the numbers exactly, you need to have some way of verifying that he got everything correct. One simple method you could employ is adding up your list of numbers and writing down the sum. Once he has copied the list, he can add his numbers up and compare his sum to yours. If the two totals do not match, it is a sure sign that he made an error. This is a very basic example of checksum verification.
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Computers and Checksums
Checksum in computers is more complicated than simply adding a list of numbers together, but it follows the same principle. In most cases, the system uses a much more complex mathematical equation known as a cryptographic hash. This produces a unique output based on the input, and computers can use hashes to verify large quantities of data very quickly. The complex nature of the cryptographic algorithm means that any small amount of variance in the data produces a significant difference in the checksum, and if the generated number does not match the expected value, you receive a bad checksum error.
Bad Checksums
The most common source of checksum errors involve your computer's BIOS. This nonvolatile portion of your computer's memory stores important information about your hardware, and your computer uses a checksum verification to ensure that the data is protected. If something happens to alter this data, your computer may report a bad checksum error when booting, requiring you to verify your settings. Repeated checksum errors may be an indication that the battery that powers this storage has failed and may require replacement.
Other Uses
Another area of computing where you may encounter checksum verification is in software distribution. Companies that produce utility programs such as antivirus and antimalware suites and make them available online have a strong interest in ensuring the authenticity of their software. In many cases, these companies publish the checksum values of their releases, and if a file fails checksum verification, it may indicate that it is counterfeit or was damaged during the download process.