Let's say you have a file. A large file. And data can corrupt over time, depending on storage medium.
Now, let's say if you looked at the data a few years later, how would you know whether the data became corrupted?
That is where the checksum comes in. If you think about it simply, without resorting to complicated algorithms, checksums does this:
it checks if the file is corrupt, based on the sum of the data.
Simple checksums calculate every byte in a data file, to get a number. Let's call this calculated checksum.
Then, on the file itself, it has the data of a previously written checksum. Let's call this written checksum.
If the calculated checksum doesn't equal to the written checksum, there's 2 reasons:
1) You used the wrong method to achieve the calculated checksum, or
2) The file is corrupt.