Biofilm
Biofilm is a slimy layer of germs that stick to wet surfaces and grow together.
Think of it this way
Grease builds up on the inside wall of a sink drain over time. Tiny bits of food and soap film stick to the wet surface first. More bits pile on top, layer by layer, until a thick, slimy coat covers the drain wall. The whole mass clings tight and is hard to scrub away.
Explaining biofilm by grade level
Think about a toothbrush that sits in a cup all week. Germs land on the wet brush and start to stick. They make a thin, slimy coat you can feel. That slime helps the germs hold on tight so water alone cannot wash them away.