A prime number is a number greater than one and can only be divided by 1 and itself. The result is always a whole number, i.e. no fractions, decimals, remainders.
Example:
2 can only be divided by 1 and itself
3 can only be divided by 1 and itself.
4 however, can be divided by 1 or 2 or itself
1 is not a prime number since it is not greater than one.