Gas has several meanings:

1. A state of matter where particles spread out to fill whatever space they're in. Air is made of gases like oxygen and nitrogen. Unlike liquids that you can pour into a cup, or solids like a rock you can hold, many gases are invisible and flow freely in all directions. When you blow up a balloon, gas fills the entire inside. When you open a bottle of perfume, gas molecules drift across the room.
2. Short for gasoline, the fuel that powers most cars. When someone says they need to “get gas,” they mean filling up their car's tank at a gas station.
3. Something hilariously funny. If your friend tells a joke that makes everyone laugh uncontrollably, you might say “That's gas!” This usage is more common in Ireland and the UK.

As an adjective, gas can describe something related to gas, like a gas stove or gas leak.