Here's why:
* Adjectives describe nouns or pronouns. In this case, "easy" describes the action of "taking it".
* Adverbs modify verbs, adjectives, or other adverbs. While "easy" can be used as an adverb (e.g., "He did it easy"), in this phrase it directly modifies the noun phrase "it".
Therefore, "easy" is functioning as an adjective, describing the state of taking things lightly.