When you fill something, you add to it until it's full, or has enough. When your car's almost out of gas, you have to fill the tank at a gas station.
You can fill a bathtub with hot water and bubbles, but there's also a figurative way to fill: "Scary movies fill me with anxiety," or "Traveling fills me with a sense of adventure." When the smell of chopped raw onions fills the air in your kitchen, your eyes might reflexively fill with tears. When fill is a noun, it means "something that fills a space."