This notation expresses a pairing between two characters of CRFH, and is usually used to express the speaker's preference towards a certain pairing. Examples: D&M (Dave and Margaret), D&B (Dave and Blue), [S&M-A], A&MT (April and [Magic Ted]), etc. Note that, since Margaret, Mike and Marsha's names all start with M, the actual meaning of 'M' in a pairing must be deduced from context (and no, not everyone likes to accept Mike and Marsha). Many times, this notation also connotes the existence of a group of persons which share the sentiment.
Sometimes this notation is also used to express a pairing between a boardie (almost always the speaker) and a roomie (as in GE&B or DrJ&M-A), but everybody knows that kind of things can't actually happen in the comic. See the [Family Tree], which includes some unofficial boardie-roomie pairs. See also [Board Societies and Coalitions].