In short
Mirror-Signal-Manoeuvre — the routine for every change of speed, direction or position.
MSM is the order to use for every change of speed, direction or position on the road: Mirror (interior, then the relevant door mirror), Signal (if it helps another road user), then Manoeuvre.
It is the single most-marked routine on the UK practical test. A missed mirror before a lane change or a turn is the most common reason learners are marked for a junction observation fault.