For learning this program you should know what are prime numbers. If i say about definition
"A prime number is a whole number greater than 1, whose only two whole-number factors are 1 and itself. The first few prime numbers are 2, 3, 5, 7, 11, 13, 17, 19, 23, and 29. "
Means, any number whose only factors (divisible by) are 1 and own number itself.
Zero and one are only numbers which we can't say prime.