The runnable interface must be implemented by any class, whose instances are intended to be executed by a thread. The Runnable interface has only one run() method.

public void run()

This method is used to perform an action for a thread.

For a detailed tutorial on Java MultiThreading, visit
https://www.c-sharpcorner.com/article/a-complete-multithreading-tutorial-in-java/

Java runnable is an interface used to execute code on a concurrent thread. It is an interface which is implemented by any class if we want that the instances of that class should be executed by a thread.

public class ExampleClass implements Runnable {

@Override 
public void run() { 
 System.out.println("Thread has ended"); 
} 
public static void main(String[] args) { 
 ExampleClass ex = new ExampleClass(); 
 Thread t1= new Thread(ex); 
    t1.start(); 
 System.out.println("Hi"); 
} 

}