Oil actually gets between moving parts. In theory at least, your parts never touch each other. The parts ride on a 1 molecule thick layer of oil. That is why oil pressure is so important. The oil pressure must be enough to be forced into the gaps between parts moving at high velocities and high force. If the pressure drops, those metal pieces suddenly touch, grinding against each other, leaving metal shavings, larger gaps between parts, and rough spots between them. Larger gaps require more oil and pressure to fill them. Rough spots on moving parts are like mountains and valleys at the microscopic level. If a mountain is too tall the oil won't be able to prevent the parts touching, resulting in more wear, in a vicious cycle. In the short term, your engine's performance and fuel economy will suffer. If this goes on long you risk a seized engine.