Some redundant bits are attached to each message to detect the existence of errors at receiving side

Basic concept:

Given a k-bit block of bits (or messages), the transmitter generates an (n-k) bit sequence, known as a frame check sequence (FCS),

such that the resulting frame (consisting of n bits)  is exactly divisible by some predetermined number.

The receiver then divides the incoming frame by that number and, if there is no remainder, receiver can assume there was no error.