Java generics work by checking types at compile time and inserting appropriate casts, but erasing the types in the compiled files. This makes generic libraries usable by code which doesn't understand generics (which was a deliberate design decision) but which means you can't normally find out what the type is at run time.
The public Stack(Class<T> clazz,int capacity) constructor requires you to pass a Class object at run time, which means class information is available at runtime to code that needs it. And the Class<T> form means that the compiler will check that the Class object you pass is precisely the Class object for type T. Not a subclass of T, not a superclass of T, but precisely T.
This then means that you can create an array object of the appropriate type in your constructor, which means that the type of the objects you store in your collection will have their types checked at the point they are added to the collection.