A superlative compares something to everything else. For example: This book is the best book there is. Here the superlative is "the best" which compares "this book" to all the other books that exist. Of course, you can narrow that and say something like: I'm the tallest person in my class. Now you're only talking about the people in your class and not everyone who exists. But, you're still doing the comparison against all the people/things in the set boundaries.
