Complex sentence: As the archive bit is a binary bit, it is either 1 or 0, or in this case more frequently called set "(1) and" clear "not changed" since the last backup.
Split into simpler sentences: As the archive bit is a binary bit, it is either 1 or 0, or in this case more frequently called "set" (1) and "clear" (0). The archive bit therefore represents one of two states: "changed" and "not changed" since the last backup.