Describe the final keyword ?

The final keyword is used in various different contexts as a modifier meaning in which what it modifies cannot be changed in a few sense.
final classes

You will notice in which a number of the classes in Java library are describe final, e.g.

public final class String

This means this class will not be subclassed, and informs the compiler in which it can perform certain optimizations it or else could not. It also provides some advantages in regard to security and thread safety.

The compiler will not let you subclass any class in which is declared final. You probably won't need or required to declare your own classes final though.