Gas Pressure
Gas Pressure is the push that trapped gas makes against the walls of its container, like the force that launches a film canister rocket.
Think of it this way
When you pump air into a bicycle tire, the air molecules bounce around inside and push against the rubber walls. The more air you add, the harder they push. That push is gas pressure — the same force that makes a balloon stretch tight when you blow it up.
Explaining gas pressure by grade level
Think about a film canister rocket. You mix baking soda and vinegar inside the canister and snap the lid on. The mix makes a gas that fills up the space. The gas pushes harder and harder on the lid until — pop — the lid flies off and the canister shoots up. That pushing is gas pressure.