Negative adverbs are those that indicate something negative about the action of the verb in the sentence. They modify the verb giving this type of information.
Some common negative adverbs are:
María no sabe que yo he vuelto.
(María does not know that I am back.)
Nunca iré allí de nuevo.
(I will never go there again.)
Jamás lo perdonaré.
(I will never ever forgive him.)
Note that when they are formed by a group of words they are called "locuciones adverbiales", i.e. "en mi vida", meaning "never":
En mi vida pensé que Javier me era infiel.
(I never thought that Javier was being unfaithful.)