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When a gas molecule bounces off of something it pushes that something. If that something is another gas molecule that other molecule goes flying off. And if that something is much bigger, for example a metal box, the molecule will push on the box just the same, but the box will hardly budge. What happens when I add more molecules to the box? all of those little bounces add up. That's pressure: the push from all the molecules. Add enough molecules and the pressure will rise so high that the box might pop!
What happens when we take out if we remove, instead of add, molecules?
Vacuum:
When the pressure is lower than the air air around us, we call it a relative vacuum.
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