Just to add a bit to the above reply, in aircraft with autostabilisers - such as most large jet transports - 'Mach Trim' is just a mode of the pitch trim system that may apply up elevator as speed increases (or vice-versa if slowing down) with the auto-stab function following up to trim out the elevator displacement. In some types the mach mode only operates when the autopilot is not in use and the aircraft is being hand flown. In this case, manual trim takes priority over mach trimming.