Answer:
Fluid-fluid level is a term that refers to the interaction between two fluids with different viscocities. This means that sonographically there would be two different gray scale level echogenicities. One of the best ways to picture this is to think of it as oil and water in the same cup, then picture that as two different gray scale levels from a sonographic point of view. Hope this answers your question and this is pretty much as simple as it gets.