this is called an "oil pump"
it uses a positive displacement vane to suck oil from the sump, and supply it at high pressure to the oil galleries of the engine, where it is transported to important places such as crank bearing surfaces, the rockers, and inside the timing cover
the oil pump is driven by a keyway that the bottom of the distributor slots into. the distributor is driven by a worm gear off the camshaft. the camshaft is driven via chain from the crankshaft. the crankshaft is put in motion either by the pistons going up and down due to combustion, or because the starter motor is spinning the flywheel, which is also joined to the crankshaft. the idea being that any time the engine is running, the oil pump should also be running
the large net thing on the bottom of the pickup is so that you can drop bolts, sockets, jewellery, etc into the sump and it won't block flow to the pump and kill the engine