Nowadays, the true meaning of parity for asynch data transmission is almost forgotten.
Error checking is often omitted at the data layer (layer 2) of the 7 layer OSI model, due to the error checking available in upper layers (3-7).
In the early days of data transmission, there were very few of the upper layer protocols (Eg IP, TCP, UDP etc) we have today to indicate the quality of a data transmission.
The term 'parity' was to ensure that there an even (or odd) number of '1' bits in each character: a simple error checing method.
The parity bit was included in the transmission of each character (now byte) as a 1-bit checksum. This was intended as a quality check on each byte to help indicate data corruption.
Parity can be:
None: means don't include the parity bit. Each character will be 1 bit shorter than if parity is included.
Mark: always insert a '1'
Space: Always insert a '0'
Even: Parity bit indicates that the data contained an even number of bits set to 1
Odd: Parity bit indicates that the data contained an odd number of bits set to 1
Note that both ends must agree on whether a parity bit is to be used, although the receiver can sometimes be set to ignore the bit, where included.