A prime number is one that cannot be split up by division. Think of 11. Twice six is 12, three fours are 12. But the only number you can divide by 11 is one, and when you have done that you still have 11 left.
Prime numbers lie at the very roots of arithmetic, and have always fascinated those concerned with figures. Choose at random 17, 23, 29, 41 take the sequence as far as you like, and you will never find a prime number divisible by another. Over the centuries the world's finest mathematicians have tried to do so and failed - although they have also been unable to prove that no such number exists.
That is because there is an infinity of prime numbers, and in theory, anything may happen in infinity. But so far the theorists have not even been able to find a rule governing the gaps between prime numbers, which is a great mathematical mystery.