Take a look at these very short sentences:
"He's tall."
"He's dead."
They both contain adjectives: tall and dead.
Tall can be used in comparative sentences:
"He's taller than me."
Dead can NOT be used in comparative sentences:
"He's deader than me."
You are either 'dead' or not; you can not be 'dead-er' or 'more dead' than someone.
Dead is therefore an example of an absolute adjective - adjectives that can not take a comparative or superlative ('dead-est') form.
Using an absolute adjective in a comparative sentences makes your English sound strange so watch out!
Another example of an absolute adjective is 'unique' because it means 'one of a kind' and, therefore, it can not be 'more unique' than something. It's either 'unique' or it isn't.
How about these words, are they absolute adjectives or not?
Link: Prepositions in phrasal verbs
