Parity is a very early error-detectiion system (now mostly replaced by CRCs). It can detect all one-bit errors, but only about half of all two-bit errors.
It consists of one extra bit per byte/word. On storage or transmission, the parity bit is set so that the total number of 'on' bits across the entire group is either even (so-called 'even parity') or odd ('odd parity'). When the data is read back or received, the parity is checked, to make sure that it is what is expected.