A prime number is the one which does not have any factor other than #1# 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#
