Mach Trim is a system that resists the nose down tendency - aka Mach tuck - of the aircraft as one Mach number increases.
This is due to the so-called *center of pressure*, i.e; the point (very theoretical point) where the lift is applied to the wing, moving aft.
Generally, usee on non-fbw aircraft on manual flight.
The 737 used to have a very noticeable onset of the Mach trim at around .70M.
On FBW-equipped airplanes, it's part of the control laws so it is quite transparent.
The rudder ratio is something quite different : It is about reducing the amount of needed rudder moment when the speed increases. So one could say, full rudder deflection only happens at low speeds.