Without getting into Professorial detail, The Internal combustion engine is a big Air pump that requires a fuel/air combustion process to keep it moving.
As the piston moves down during the intake stroke, it creates a LOW pressure condition which draws in (atmospheric pressure) based on the differential of HIGH to LOW. Within the intake system there is a Throttle plate that causes a measured restriction in airflow. Upstream of the throttle is the Filtered Intake or air inlet. Downstream of the plates is the Upper deck (manifold pressure side). When the pressure is LOWER on the Manifold side of the throttle plates it creates a vacuum that can be utilized for auxiliary purposes ( all vacuum ports) are downstream of the plates. When the throttle plates are closed you are at maximum differential, creating strong vacuum. Engine RPM plays into this too. Think high RPM deceleration. Attached too the manifold side of the intake is the emissions system, evaporative fuel vapor and Crankcase vent (Blow by recirculation) and a vacuum tank storage system with a CHECK VALVE too smooth out the transitional process between open throttle and closed.
Note: back in the OLD days before the invention of electric windshield wipers, they were vacuum powered. I always thought it was funny when driving my Uncles 1936 Dodge pick-up to a car show and having to drive through rain showers during the trip. Because there was no storage tank system to draw from, when you step on the gas pedal to accelerate or climb a hill and the one arm wiper stops moving due to loss of vacuum (throttle plates open). let off the gas to coast and it's back in action.