Plain English Meaning

It is the top speed an aircraft is allowed to fly. The limit is shown two ways — as a regular airspeed for lower altitudes, and as a Mach number for higher altitudes — because which one matters more depends on how high you are.

Full Definition

VMO/MMO is the maximum operating limit speed of an aircraft, expressed two ways: VMO as an indicated airspeed (in knots) and MMO as a Mach number (a fraction of the speed of sound). The aircraft must not be flown faster than VMO or MMO, whichever is the limiting value at the current altitude. At lower altitudes the indicated airspeed limit (VMO) is the active limit; at higher altitudes the Mach limit (MMO) becomes the active limit because the speed of sound decreases as the air gets colder with altitude.