Checksum is the result of a function that was applied to a number, string, stream (a data entity in general) in order to add it for integrity issues. One simple example of a checksum is the parity bit, which mostly is calculated as follows:
The binary number 10110 contains 3 times 1. 3 is odd-numbered (it is of "odd parity"), so the parity bit is 1.
The binary number 10111 contains 4 times 1. 4 is even, so the parity bit is 0.
Please note, that this also may be the other direction, so that the parity bit is set to 1 to indicate an even number.