Technically speaking, the pump creates a vacuum in the tube, just above the oil-level, and the surrounding (crankcase/atmospheric) air "pressure" forces the oil into that low pressure area, which feeds the oilpump.
The level of the oil in the pan pushes the oil up the tube to a balanced level with the oil in the pan.
The suction or vacuum created by the input side of the pump draws the oil from that level in the pan up into the pump.
