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Balloon pump
A hand pump is a piston in a cylinder with two one-way valves. Each time you push it down it sends a barrel full of air into the balloon.
Barrel and plunger
The handle, rod and piston move together as the plunger. The piston's rubber seal fits the barrel tightly, so the air can't slip past it.
Pull up: air comes in
As the piston rises, the space below it gets bigger and the pressure drops. Outside air pushes the inlet flap open and fills the barrel, and the outlet valve stays shut.
Push down: air goes out
Pushing the piston down squeezes the trapped air. The higher pressure slams the inlet flap shut and forces the outlet valve open, so the air rushes down the hose.
The balloon fills
Each stroke adds more air. The outlet valve stops it flowing back, so the pressure builds and stretches the rubber until the balloon is full.