Okay kiddo, so when we talk about "available light", we're talking about the light that's already around us. It's not any kind of special light that we bring in, like a flashlight or a lamp. It's just the light that's coming from the sun, the moon, or any other source that's already there.
Think about when you're outside on a sunny day. You don't have any extra lights with you, right? But you can still see just fine. That's because there's plenty of available light from the sun.
But sometimes, there might not be enough available light for us to see well. For example, if we're inside a dark room with no windows, or if it's nighttime and there's no moon or streetlights. In those cases, we might need to bring in some extra light - like turning on a lamp or lighting a candle.
So "available light" just means the light that's already there, without us needing to add any extra light sources.