A prime number is a number that only has two factors (the number 1 and itself).
12 has more than two factors, so it is not prime.
31 only has two factors, so it is prime.
99 has more than two factors, so it is not prime.
1 only has one factor, and although it is commonly debated as to whether or not it is prime, since it only has one factor, it is not considered a prime number (although in the 19th century it actually was considered prime, it is not anymore).
