Well, biomethane is a type of gas that can be made from poop, and other organic materials like food waste and grass.
You see, when things like poop and food waste break down, they create something called methane gas. That's a gas that can be used as a fuel, like for a car or stove. But sometimes, this gas goes into the air and makes the air dirty.
So what people can do is collect this gas, clean it up so it's not dirty anymore, and turn it into something called biomethane. This has no bad smells or icky germs in it, and it's much better for the environment than other types of fuel.
Some people even use biomethane to help make their homes or businesses run on renewable energy. So, biomethane is kind of like a very clean and nice-smelling gas made from the things we'd normally throw away. Cool, huh?