Computers use "clocks" (essentially, simple pulse generators)
to step the microprocessor from one line of program (and data) to the next.
The faster the sequence of pulses, the faster the processor does its job.
Each different type of processor has a maximum speed at which it can be used,
but it can be run at any slower speed, or, with special provisions, stepped manually,
if there is some reason to do that.
3 MHz is 3 million pulses/steps per second, sorta slow compared to current speeds,
but much better than the few thousand per second of some of the earliest konfuzers.