__Prime numbers__ are numbers that have exactly 2 divisors (numbers you can divide by and still have a whole number as a result)

2, 3, 5, 7, 11, 13, 17, ... are prime numbers, because they only have 1 and itself as divisors

12 is not a prime number, because it has 1, 2, 3, 4, 6 and 12 as divisiors.