The complete sentence should be:
Composite numbers can be written as a product of prime factors. This is called the prime factorization of a number.
A prime number is a number that can't be divided by any other number other than 1 or itself. Otherwise, that is a composite number. For example, 50 is a composite number. Through prime factorization,
50
/ \
10 5
/ \
5 2
The prime factors of composite number 50 are 5, 5 and 2.