A complex fraction is a fraction that is itself part of a fraction. They look something like a/b / c/d. The middle slash is the line that divides the fraction which constitutes the numerator of the complex fraction from the one that constitutes the denominator of the complex fraction. All we do is invert the latter and multiply it by the former. We are left with ad/bc. Let's see how this works.
a/b / c/d = a/b times d/c = ad/bc. Notice that a/b is multiplied by d/c, which is an inversion of the denominator of the original complex fraction. This is all we do to simplify the complex fraction. Let's look at another example to ensure that we've understood the concepts.
x/y / w/z = x/y times z/w = xz/yw. Notice once again that x/y is multiplied by z/w, which is an inversion of the denominator of the original complex fraction. As before, this is all we have to do in order to simplify the complex fraction.