This allows for simple aliases to be created for user mailboxes by appending +text to the users address.
For example the user bob@domain.com can have aliases of
bob+marketing@domain.com,
bob+sales@domain.com,
bob+anythingyouwant@domain.com.
These aliases can then be combined with mailbox rules to filter the messages. This permits users to create different accounts in online sites and reduce the potential for any email/password breaches.