What is an adverb?

Adverbs are used to modify verbs. 
They tell us when, where, how, in what manner, or to what extent an action is performed.

Some examples: 
When: He ran yesterday.
Where: He ran there.
How: He ran quickly.
In what manner: He ran barefoot.
To what extent: He ran the fastest.

In these examples the adverbs are just one word, but they can be made up of more than one word.