What is an archive bit?

What is the meaning of "archive bit?"

It is an attribute of a file, for example, the DOS command ATTRIB would display the attributes of a file such as read-only, hidden, system file, etc. It indicates whether a file was backed up since it was modified. Some backup products use this file attribute to determine if a file should be backed up or not. However, many enterprise backup products now use their own database or catalog to track file changes.