What concept does the above sample code demonstrate?

internal class Piston {}
internal class Engine
{
private Piston[] myPistons = new Piston[4];
public bool Ignition() {
//some code
}
}
public class Car
{
private Engine myEngine = new Engine();
public void Start()
{
//put in keys etc..
if (myEngine.Ignition()) {
//some more code

}
}
}