A prime number is the one which does not have any factor other than 11 and itself. For example, 2,3,5,7,11,13,17,19,.... do not have any other number as a factor other than 1 and themselves - for example only factors of 13 are 1 and 13 itself.
Expressing a number as a product of prime factors means writing it as a product of all numbers which are prime. If in factorization, we get a non-prime as factor we divide it further, so that they are prime.
For example 390=10xx39, but 10 and 39 are not prime numbers and have factors 2xx5 and 3xx13 respectively and hence 390 as a product of primes is 2xx5xx3xx13 or writing numbers in increasing order 2xx3xx5xx13. (This way of prime factorization is called tree method.) Observe that all these numbers 2,3,5,13 are prime numbers and cannot be factorized further.
As 13 is a prime number it can be written only as 13=13
