Well, that is a loaded question.
There is a somewhat simple answer and it may help you understand the basics of most automobiles made since the late 1980’s.
As most of us know, an internal combustion engine needs three things to run. Air, fuel and spark. Prior to the days of computers, fuel was provided via a mechanical (or sometimes electric) fuel pump to a carburetor. The natural vacuum (technical term is manifold pressure) would draw the fuel into the engine via Bernoulli’s principle. The air fuel mixture would be compressed in the cylinder and ignited by the spark plug. You probably know the rest of the story. If not look here.