A prime number is any natural number that has exactly two natural divisors, itself and 1.

Examples:
2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31 ...

4 is not a prime number because it has more then 2 divisors: 1, 2 and 4.

6 is not a prime number, it has 4 divisors: 1, 2, 3, 6.

2 is the only even prime number.