What is the synchronized method in Java

Question

What is the main idea of synchronized method and synchronized block in Java?

And why should we use them?

Sample code would be nice. I have read the Java documentation about synchronized method, but I didn't get the idea.

This is the Java documentation code

public class SynchronizedCounter {
    private int c = 0;

    public synchronized void increment() {
        c++;
    }

    public synchronized void decrement() {
        c--;
    }

    public synchronized int value() {
        return c;
    }
}