An internal combustion engine includes many components that require a supply of oil in order to operate most effectively.  Oil may be provided in order to lubricate movement of the components and/or cool the components. 
Oil that has been delivered to the components drains back to an oil sump of the engine assembly and is stored in the oil sump until it is pumped back to one of the oil consuming components of the engine. 
An oil pump is often provided close to the oil sump of the engine and is configured to pump a supply of oil to each of the oil consuming components of the engine.  The oil pump is typically a mechanical pump driven by a shaft of the engine.  The oil consuming components may require a high pressure and/or flow rate of oil and hence the oil pump may draw a significant amount of power from the engine in order to operate.