Catch-All email addresses will allow you to receive email sent to firstname.lastname@example.org that is not already a specified address. You may create a catch-all address by creating an email address named default i.e. email@example.com.
Otherwise email sent to unknown users @example.com will have a unknown user bounce sent back to the sender.
Use of the Catch-All is *not* recommended as it will increase the level of spam delivered to your mailbox tremendously.