Here's why:
* It modifies a verb: It tells us *when* something is happening.
* Example: "We're going to the movies tonight." (Modifies "going")
* It can also modify an adjective or another adverb:
* Example: "It's tonight that we feel the most alive." (Modifies "tonight" itself, acting as an adverb of time)
Let me know if you'd like more examples!