In a simple to understand explanation, what is Runnable in Java?

A Runnable is basically a type of class (Runnable is an Interface) that can be put into a thread, describing what the thread is supposed to do.

The Runnable Interface requires of the class to implement the method run() like so:

public class MyRunnableTask implements Runnable {
     public void run() {
         // do stuff here
     }
}

And then use it like this:

Thread t = new Thread(new MyRunnableTask());
t.start();

If you did not have the Runnable interface, the Thread class, which is responsible to execute your stuff in the other thread, would not have the promise to find a run() method in your class, so you could get errors. That is why you need to implement the interface.

Advanced: Anonymous Type

Note that you do not need to define a class as usual, you can do all of that inline:

Thread t = new Thread(new Runnable() {
    public void run() {
        // stuff here
    }
});
t.start();

This is similar to the above, only you don’t create another named class.