Checksum is a code used to verify data. It is created by performing a mathematical operation on all the data together, and will change according to the value of the data.
Checksums are used to make transmitted data is correct. When sending data, the transmitter calculates the checksum of the data that is sent, and sends it with the data. The receiver also calculates the checksum of the data it receives. If the checksums don't match then the data�must be�different - i.e. it has not been�correctly transmitted�- and needs to be re-sent.