When referring to a computer processor, MHz is short for megahertz and is one million Hertz. An oscillator circuit supplies a small amount of electricity to a crystal each second that is measured in kHz, MHz, or GHz. "Hz" is an abbreviation of Hertz, and "k" represents Kilo (thousand), "M" represents Mega (million), and "G" represents Giga (thousand million).
The first computers processors (CPU's) operated in kHz. For example, the first processor, the Intel 4004 operated at 740 kHz. Later processors operated in MHz, for example, the Intel Pentium processor was available in speeds of 60 MHz to 300 MHz. Today's processors operate in the GHz. When measuring the speed of a processor, the higher the speed (bigger the number), the faster it is capable of running.