Tire Pressure
Tire Pressure is how much air pushes against the inside walls of a tire.
Think of it this way
A sealed jar holds air inside it. The more air you pump into it, the harder that air pushes against the glass walls. You can feel the walls flex slightly when the pressure is high. Tire pressure works the same way — air packed inside pushes outward against the rubber walls from every direction.
Explaining tire pressure by grade level
A car tire is full of air. That air pushes on the inside of the tire. More air makes the tire feel hard. Less air makes it soft and flat. A soft tire drags on the road and makes the car work harder to move.