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Your engine sucks in air as the piston moves down. Same way as you suck in air as your diaphragm contracts to make your lungs more voluminous.
Gasoline engines run at a fairly fixed air fuel ratio. So every bit of air has a bit of gasoline injected to it and the ratio is constant.
So the wat the engine power/speed is controlled is by a valve to limit the incoming air. (Valve is connected to the gas pedal.
You can imagine this as you wanting to run like Forest Gump but some one controls your speed by closing your mouth.
Obviously trying to breathe with a semi closed mouth is not optimal because you have to suck in air through a restriction which wastes energy. Same applies to an engine. The vacuum gauge is measuring the air pressure in your mouth. The less it is, the more efficiently the engine runs.
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