good - this word is an adjective that modifies a noun.
in the sentence, "his salary is very good" it modifies the noun salary.
well - is an adverb that modifies a verb,adjectives and another adverb.
adverbs often answers the question "how"
i can speak Filipino well. [how do i speak Filipino? - the answer is well.]
in this sentence, the adverb well modifies the verb speak