If the case for either does not hold true, the receiving end sends a request to retransmit the stream of bits. They are retransmitted, with the parity bit attached all over again. For example, a computer sends a bit stream of 10101011. This is an odd number so add a 0 to the end of the stream to make it 101010110. The bits are received at the other end, added together, and compared to the parity bit the same way.
Parity check adds or sums the bits in a byte.
If it is an odd number a 0 is tacked onto the end of the group and if it is an even number a 1 gets put on. Once received, Mr. Computer adds up the original eight bits. It then looks at the parity bit, to see if it agrees with the sum.