An adverb is a part of speech used to describe a verb, adjective, clause, or another adverb. It simply tells the readers how, where, when, or the degree at which something was done.
The manager accepted the challenge very "nicely."
The italicized word is an adverb that describes nicely, which is another adverb.
Tears began to fall as he saw the completely lifeless body of his wife.
The adverb in this sentence is completely, which describes the adjective" lifeless."