It's not required. It's a hint to the compiler that the method is intended to override some other method. That allows the compiler to check that it actually does override some method. Otherwise, a typo in the method name or incorrect parameter types would lead to no method being overridden - and you'd be left wondering why your method doesn't get called. Also see http://download.oracle.com/javase/tutorial/java/javaOO/annotations.html
If your books don't cover it that may be a sign that they're somewhat old - it was introduced only with Java 5.
To give you a concrete example: Consider the hashCode() method of class Object. Sometimes you want to override that method in your own classes. But it's easy to forget that the method is called hashCode(), and not hashcode() with a lower-case c. When you do this:
You will get a compiler error, because the compiler notices that your hashcode() method is not overriding any method from a superclass - so it helps you to catch your error.