An adverb is a word that modifies a verb, an adjective, or another adverb. Among the many things adverbs may express we have intensity, frequency, place, time, etc.
In the sentence "Her coach had a <u>very </u>nervous expression on his face," the adverb is "very". In this sentence, "very" is modifying an adjective, "nervous". The purpose of using "very" is to intensify the meaning of the adjective. The coach was not simply nervous, he was very nervous.
