According to Wikipedia, the 90 / 10 rule of program optimization states that “90% of a program execution time is spent in executing 10% of the code” (see the second paragraph here).
I really don't understand this. What exactly does this mean? How can 90% of the execution time be spent only executing 10% of the code? What about the other 90% of the code then? How can they be executed in just 10% of the time?