Define computer speed and give examples?

The relevant "speed" is basically, how quickly the computer can get work done. There is no precise way to define this; but one measure that is often used is called "flops" - floating-point operations per second. That means additions, subtractions, multiplications, or divisions.
As an example, in 2016, the #1 supercomputer (according to the "Top 500" list) can do approximately 93 petaflops - that is 93 x 10 to the power 15 operations per second.