When your guest goes to RSVP and enters their first and last name, the system will search for an exact match. If it doesn't find an exact match, it will then return all parties containing a last name-only match.
In other words, if your guest enters "John Doe" and there is a party containing the name "John Doe" in your Guest List ONLY this party will be displayed to the guest. If, however, your guest enters "Johnny Doe" (and the system does not find an exact match) the system will return ALL parties in your Guest List containing a last name of "Doe."
This configuration ensures that guests with variations on their first name are still able to find their party if they enter it differently than you, the event's host, has entered it in your Guest List.
(NOTE: If 'Require exact match name entry' is enabled in Guest List > Settings, BOTH the guest's first AND last names must be an exact match to a guest on your guest list.)