Adverbs of quantity are those that talk about the quantity (how much) in relation to the action. They modify the verb giving this type of information.
Some common adverbs of quantity are:
Te quiero mucho.
(I love you lots.)
Aquí hace bastante calor.
(It is quite hot in here.)
Tengo que comprar más mantequilla.
(I need to buy more butter.)
Tengo mucha sed. Dame un poco de agua, por favor.
(I am thirsty. Give me a bit of water, please.)
Note that when they are formed by a group of words they are called "locuciones adverbiales", i.e. "un poco de".