Perforations in a stamp are a series of holes punched out between stamps for easy separation. (Stamps that come in sheets with no perforation are said to be imperforate).
To determine the the number of perforations in a stamp the use of a perforation gauge is recommended. The numbers referred to as sizes are actually the number of perforation holes that fall in a space of two centimeters. Thus if a stamp is said to be perf 10 it is one that has 10 perf holes in the space of two centimeters.
An identical stamp may have two diff perf, leading one to be more scarce & of greater value than the other. Some stamps have a different measurement on the sides than they do on the top & bottom; these are called compound perfs, and the first number will be the top measurement - perf 11x10, for example.