Why A parity bit is one extra bit appended to each character, set so the total number of 1 bits is always even (or always odd, depending on the convention). The receiver counts the bits and flags the character if the count no longer matches the expected parity, which reveals a single bit flipped in transit. That is error detection only, not correction, and it misses errors that flip an even number of bits.
Watch out Adding a parity bit actually slows throughput slightly because you send 8 bits instead of 7 for the same character, and it does nothing to the signal-to-noise ratio or the size of the character set.
Parity = one spare bit that says 'count my ones.' It detects errors, it does not fix them or add speed.