Prime numbers are those numbers which cannot be divided evenly by any number other than 1 and itself.
For example, 12 is not a prime number, because it can be divided evenly by numbers other than 1 and 12 (namely, 2, 3, 4, and 6).
On the other hand, 13 IS a prime number, because it can only be divided by 1 and 13 evenly.
Just as a sidenote, numbers that are not prime numbers are called 'composite numbers'.
Hope that helped =)