What is sub-addressing?

The so-called sub-addressing / mail tagging / plus-addressing (according to RFC 5233) is a special formatting in e-mail addresses, which encodes several sub-functions into an e-mail address.

These can automatically trigger certain actions on receipt, which are freely configurable.

An example:

Your actual email address is peter@mydomain.com. You can now log in to Facebook and enter the email address peter+facebook@mydomain.com there.

Every e-mail message to this address will be received normally in your mailbox peter@mydomain.com and will automatically trigger the action you set for subaddresses (move to subfolder or extend subject line).

See also:
Wikipedia: Subaddressing