Official Aviation Definition
Mach number (named after physicist Ernst Mach) is a dimensionless quantity representing the ratio of an aircraft true airspeed to the local speed of sound. Because the speed of sound decreases with lower air temperatures, Mach number varies with altitude and ambient temperature.
In Simple Words
Your speed compared to the speed of sound. Mach 0.80 means you are traveling at 80% of the speed of sound.
Real-World Flight Example
Long-range commercial airliners like the Boeing 777-300ER and Airbus A350 typically cruise between Mach 0.82 and Mach 0.85.