Some additional detail to what Dennie is saying:
There are gradable adjectives (which can be modified to express their intensity)
I am tired, I am very tired, I am extremely tired
There are non gradable adjectives (which cannot be modified since they're already extreme)
This is amazing, This is terrific, This is fabulous, I'm exhausted
Non gradable adjectives can be:
classifying adjectives: married, wooden pregnant
extreme adjectives: brilliant, excellent, disgusting
absolute adjectives: dead, supreme, final
And finally there are gradable/non gradable adjectives, that is, adjectives that sometimes behave as graded and sometimes as non graded:
She sounded slightly foreign to me (it's graded here by "slightly")
He is working on foreign policy (cannot be graded)
Note that many non gradable adjectives can nevertheless by the adverb "really" - I would say just the "extreme" ones not the other two. I hope this helps.