A prime number can be divided, without a remainder, only by itself and by 1. For example, 13 can be divided only by 13 and by 1.
Some facts of prime number:
To prove whether a number is a prime number, first try dividing it by 2, and see if you get a whole number. If you do, it can't be a prime number. If you don't get a whole number, next try dividing it by prime numbers: 3, 5, 7, 11 (9 is divisible by 3) and so on, always dividing by a prime number (see table below).