Okay. It's called cross-multiplication, because you have something like this:

Now, you multiply both sides by b, and you multiply both sides by d. Effectively, you bring the b to the RHS, and the d to the LHS.

That is why it's called cross multiplying, because you're taking across the denominators to the other side (by multiplication).

The "cross product" is a completely different operation. Don't worry about it.