What is meant by a multithreaded program in Java?

A multi-threaded program contains two or more parts that can run concurrently and each part can handle a different task at the same time making optimal use of the available resources specially when your computer has multiple CPUs.
Multi-threading enables you to write in a way where multiple activities can proceed concurrently in the same program.

Example

class RunnableDemo implements Runnable {    private Thread t;    private String threadName;       RunnableDemo( String name) {          threadName = name;          System.out.println("Creating " + threadName);       }       public void run() {       System.out.println("Running " + threadName);       try {          for(int i = 4; i > 0; i--) {             System.out.println("Thread: " + threadName + ", " + i);             // Let the thread sleep for a while.             Thread.sleep(50);          }       } catch (InterruptedException e) {          System.out.println("Thread " + threadName + " interrupted.");       }    System.out.println("Thread " + threadName + " exiting.");    }    public void start () {       System.out.println("Starting " + threadName );       if (t == null) {          t = new Thread (this, threadName);          t.start ();       }    } } public class TestThread {    public static void main(String args[]) {       RunnableDemo R1 = new RunnableDemo( "Thread-1");       R1.start();       RunnableDemo R2 = new RunnableDemo( "Thread-2");       R2.start();    } }

Output 

Creating Thread-1 Starting Thread-1 Creating Thread-2 Starting Thread-2 Running Thread-1 Thread: Thread-1, 4 Running Thread-2 Thread: Thread-2, 4 Thread: Thread-1, 3 Thread: Thread-2, 3 Thread: Thread-1, 2 Thread: Thread-2, 2 Thread: Thread-1, 1 Thread: Thread-2, 1 Thread Thread-1 exiting. Thread Thread-2 exiting.