Atmospheric Pressure
Atmospheric Pressure is the weight of air pushing down on everything around you.
Think of it this way
A stack of heavy books sits on top of a flat tray. The books press down on the tray from above. Air works the same way. It piles up miles high, and all that weight pushes down on everything below. The higher you go, the less air sits above you, so the push gets weaker.
Explaining atmospheric pressure by grade level
Air has weight, even though you cannot see it. All that air above you presses down on things. When you go up a tall mountain, there is less air above you. With less air pushing down, water boils at a lower heat.