An adverb (adverbio in Spanish) is a word that modifies the meaning of verbs, adjectives or other words. 

E.g. "The girl smiled nervously"- the word "nervously" describes how she smiled.

There are two ways of forming an adverb:

1)  The majority of Spanish adverbs are formed by adding -mente to the (feminine singular form of the adjective) end. Such ending corresponds to -ly in English. 

E.g. "Oscar escribió lentamente" (Oscar wrote slowly)

2)  The other way to form an adverb is to use the preposition con + the singular form of the noun. 

E.g. "Oscar escribió con cuidado"

Note: When an adjective has a written accent (tilde), the adverb retains it. E.g. "Oscar escribió rápidamente" (derives from the adjective "rápido").