A prime number is a number that has no factors, except for itself and one.
21 is not a prime number, because in addition to having one and itself as factors, it has 7 and 3. ( 1 x 21 = 21.) ( 7 x 3 = 21.)
22 is not a prime number. It's easy to tell because it is an even number. ( 1 x 22 = 22.) ( 2 x 11 = 22.)
24 is not a prime number. It's even, but also has more factors. ( 2 x 12 = 24.) ( 3 x 8 = 24.) (4 x 6 = 24.)
23, however, has no other factors but one and itself. ( 1 x 23 = 23.) So therefore, it is prime.