Check sums are a type of error-detection code used to detect errors in data transmission. They are calculated by adding up the values of all the bytes in a message and then sending the result as an additional byte. This allows the receiver to CHECK that the data has been received correctly. If the sum of the bytes does not match the CHECK sum, then the data is assumed to be corrupted and the receiver can request a retransmission. The term 'CHECK sums' is derived from the words 'CHECK' and 'sums', which refer to the process of verifying the accuracy of the data.