The engine in your vehicle uses oil to lubricate all of its moving parts inside. As the oil passes over and through these components, it also cools them down by absorbing heat. When the oil becomes too hot, however, it begins to break down or degrade. At that point, it no longer protects your engine the way it should.
To prevent excessive heat from building up in the engine oil, some vehicles come equipped with an oil cooler. The oil cooler is usually an aluminum box with small or passageways inside of it. Some of these passageways carry oil. Others carry engine coolant. A thin layer of aluminum separates the two liquids. Oil cooler lines carry the oil and coolant to and from the cooler.
When oil passes through the oil cooler, some of its absorbed heat is transferred to the engine coolant. And, just as is the case with the rest of the engine cooling system, the coolant flows to the radiator where the heat dissipates into the atmosphere.