A Prime Number can be divided evenly only by 1, or itself.
And it must be a whole number greater than 1.
Example: 5 can only be divided evenly by 1 or 5, so it is a prime number.
But 6 can be divided evenly by 1, 2, 3 and 6 so it is NOT a prime number (it is a composite number).
See: Composite Number