Stopwatch System class in java

Could you please tell me, what is the "System. " in this code? and why they used it? when we should use "System. "? Where can I know I should use System. for nanoTime()?

 // A class to measure time elapsed.   
  public class Stopwatch{
    private long startTime;
    private long stopTime;

    public static final double NANOS_PER_SEC = 1000000000.0;

     // start the stop watch.
    public void start(){
    startTime = System.nanoTime();
    }

    // stop the stop watch.
    public void stop()
    {   stopTime = System.nanoTime();   }


    // elapsed time in seconds.
    // @return the time recorded on the stopwatch in seconds
    public double time()
    {   return (stopTime - startTime) / NANOS_PER_SEC;  }

    public String toString(){
        return "elapsed time: " + time() + " seconds.";
    }

    // elapsed time in nanoseconds.
    // @return the time recorded on the stopwatch in nanoseconds
    public long timeInNanoseconds()
    {   return (stopTime - startTime);  }
}