The "M" starts your name - "Mrs.", "Mr.", "Miss", or "Ms.". So if you invited me to your wedding I would write "r. and Mrs. William Carr" on that line. Then I would check "accepts with pleasure" :). It's also considered socially acceptable to use "Name(s)" in place of "M". This is usually reserved for less formal weddings but I don't write rules like that in stone.
Hope that helps! Have a wonderful Wednesday!