A shortening of the already abbreviated 'RSVP Regrets Only'. When you see this on an invitation, it means that you need only reply (RSVP) if you are not coming to the event. Etiquette does not require that you give any sort of excuse or reason for not coming. It does require that you respond if you are not going to attend; on the other hand, if you are going to attend, it is improper to respond to a Regrets Only.
Sometimes this is shortened yet again, to just 'regrets'.