I have never RSVP'd to a wedding. The only weddings I have been to have been family. The question is: on the RSVP note what does the "M" stand for and do I write our names on that line?

The M allows the guests to write in Mr., Mrs., Miss, Ms, etc. and their names indicating that they will (or will not) attend the wedding. The guests then return the RSVP card to the bride (in the self-addressed, stamped envelope provided by the bride along with the RSVP card). This way the bride can have an accurate count of how many people are coming - which the bride will need for the reception.