The divisor is a factor if you get the remainder zero after division.
Just as you know that 4 is a factor of 16 because 16/4 has zero remainder.
For example,
(2x-1)(x+2)(3x+7) / (x+2)
= (6x^3+23x^2+15x-14)/(x+2)
we know that the remainder is zero, because we constructed it that way. To make that a fraction where the divisor is NOT a factor, just change any of the coefficients. Then the divisor will no longer go in evenly.