Basically it means to reduce or make the denominator into the smallest possible numbers that it can be, basically ones, that can't divide into other numbers other than 1 and itself.
For example if the denominator is 12.
The product of numbers that multiply to give 12, can be 4 and 3, yet we can still divide 4 by 2, 4 and 1, NOT only 4 and 1, thus we need to reduce 4 into other numbers that multiply to give it, which would be 2 • 2, here we have both numbers, 2, that are prime numbers or numbers that can be divided by 1 and itself.
Thus the prime factorization of the denominator 12 in this case would be :
12 = 2 • 2 • 3.