An adverb is a word that modifies a verb. For example, if I say Lizzy runs, the verb is run and the subject is Lizzy. There is no adverb. But if I say, Lizzy runs fast. Now I have included an adverb. Can you guess what it is? It's the word RUN, because it modifies or describes or tells you something more about the verb. Before, all I knew was that Lizzy runs. But now I know that she runs FAST.
Here's another example. John studies for his test. So far, there's nothing special about that sentence. All that we know is that John studies for his test. But, if I say John studies hard for his test, then I added something that describes HOW he studies. He studies HARD.
Think of it this way. When you ADD something to a VERB, you have an ADVERB.