Adjectives describe qualities (characteristics) of nouns. Some qualities can vary in intensity or "grade", for example: rather hot, hot, very hot; hot, hotter, the hottest
ex : The adjective hot is gradable.

Other qualities cannot vary in intensity or grade because they are:
1. extremes (for example: freezing)
2. absolutes (for example: dead)
3. classifying (for example: nuclear)

The adjectives freezing, dead and nuclear are non-gradable.