Gradable adjectives describe qualities that can be measured such as age, size, beauty, etc. They can be used in comparative and superlative forms and with grading adverbs such as very, quite, extremely, etc.
Non-gradable adjectives describe qualities that are completely absent or completely present.
eg. Spanish, blue, chemical, tasteless, divorced, perfect, impossible, dead, etc.
Non-gradable adjectives cannot be used in comparative and superlative forms.