An Adverb qualifies any part of speech except a noun or pronoun, but it usually qualifies a verb, and adjective, or another adverb.
( a ) He runs fast
( b ) He runs very fast
( c ) He is quite happy.
In ( a ) the adverb ” fast ” qualifies the verb ” run ” ;
in ( b ) the adverb ” very ” qualifies the adverb ” fast ” ; and
in ( c ) the adverb ” quite ” qualifies the adjective ” happy “.