From SKYbrary Wiki
A machmeter is an instrument which provides an indication of the Mach Number, (M), which is the ratio between the aircraft true air speed (TAS) and the local speed of sound (LSS). This ratio, which equals one when the TAS is equal to the local speed of sound, is very important in aircraft operating at high speed.
The machmeter uses the aircraft pitot-static system to generate M and usually portrays this on a simple needle and dial instrument, such as that shown below.
Alternatively, the machmeter may be combined with the Air Speed Indicator (ASI), in which case it is often referred to as a Combined Speed Indicator (CSI).
High speed aircraft, including airliners and business jets, have limiting mach numbers which must not be deliberately exceeded. If the aircraft is deliberately or accidentally allowed to exceed its limiting mach, shock waves are likely to form on the aerofoils and can result in buffet or mach tuck.
Some aircraft use a constant mach number (rather than constant speed) technique for cruise operations. Constant mach technique may be used to separate aircraft on the same track and at the same altitude whilst in a non radar environment.