Checksum is a method to make sure a file was transmitted correctly.
Every single character in a file has a value. You can add all of them, write down the result, then transmit it, calculate sum of downloaded file and compare them. If they're equal, the file is OK, if not, there was a transmission problem (and you should download again).
The sums you're using are not simple sums, they're more complicated, but the idea is as above.