What is extreme adjective?
As I have known:
An extreme adjective is the non-gradable form of a gradable adjective.
Ex: Freezing = very cold, furious = very angry, great = very good.
You can know more about it here: http://english-learners.com/2009/10/extreme-adjectives.html
I found this link helpful.
Here's an explanation from the internet ( http://www.ecenglish.com/learnenglish/lessons/extreme-adjectives ):

Starving' is an extreme adjective, it means very, very hungry. We have lots of extreme adjectives in English that are used to emphasise normal adjectives.

Here are nine examples of adjectives and their extreme adjectives.

Adjective - Extreme Adjective
1. Dirty - Filthy
2. Funny - Hilarious
3. Clean - Spotless
4. Interesting - Fascinating
5. Awful - Dreadful
6. Ugly - Hideous
7. Tasty - Delicious
8. Big - Huge
9. Small - Tiny
Extreme adjectives are different from normal adjectives in that they do not normally take the intensifier very: e.g. very good, very brilliant
Extreme adjectives use other intensifiers such as absolutely, completely, totally, utterly. These intensifiers are not usually used with normal adjectives: e.g. absolutely brilliant, absolutely good