There are very few stupid questions, yours isn't one of them. In the context of aquarium keeping, Biofilm is a collection of bacteria, diatoms, algae, fungi and other multi-cellular organisms that form a layer on any surface submerged in water (including seawater). Biofilms form because macro-molecules (e.g. Sugars, proteins) attach to surfaces because surfaces (at the molecular level) are polar (i.e. have positive and negatively charged areas). And bacteria are the first to attach to these surfaces to make use of these molecules. The bacteria make the surfaces attractive for settlement of other organisms. Each surface also has a unique biofilm depending on what molecules, bacteria or other organisms attach to it. This is more than the average shrimp keeper needs to know. The important thing shrimp keepers need to know is that shrimp eat this biofilm and it forms an important part of their diet. Hence why we feed our shrimp IAL and similar leaves, because as these leaves break down their surfaces are colonised by micro-organisms which the shrimp eat.