Here's why:
* It modifies an adjective or another adverb: It tells us how much or to what extent something is.
* Example: "The room was very warm." ("Very" modifies the adjective "warm")
* Example: "She ran very quickly." ("Very" modifies the adverb "quickly")
* It doesn't modify a noun: Adverbs don't describe nouns; adjectives do.
Let me know if you'd like more examples!