Hi kiddo, have you ever heard about fermentation? It's like when you make pickles or bread and it turns sour and chewy. Now imagine two different types of living things (like animals or bacteria) helping each other to ferment things in their shared home, that's symbiotic fermentation.
One of these living things, let's call it A, eats some food and turns it into something that the other living thing, B, likes to eat. But when A eats food, it releases gas that B can't use. So B eats that gas and turns it into something else that A likes to eat! This back and forth process keeps happening as the two living things break down and change the food they're eating in a special way.
This is like a perfect match or friendship! They help each other survive and get what they need to make energy so they can keep living happily ever after, just like you and your best friend!