Adverbs modify a verb, an adjective or another adverb. Adverbs of quantity express the idea of how much or in what degree.

Some examples:

muy - very
mucho - much
poco - a little, a bit
algo - somewhat
nada - nothing
mitad - half
demasiado - too much

Let's see a few example sentences:

Tú vas a llegar muy tarde.
You will arrive very late.

Usted no ha hecho nada malo.
You have done nothing wrong.

Esta casa la ha vendido; esa era una idea bastante tonta.
He has sold this house; that was a quite foolish idea.

Aquí casi nunca llueve.
It hardly ever rains here.

La casa unifamilar es mucho más grande.
The townhouse is much bigger.