An adverb is a word that modifies a verb. An adverb clause is a group of words which acts as an adverb in a sentence.
Adverb clauses can add explanatory detail to ones writing and explain how things happened. Adverb clauses elaborates on why, when, where and how a particular situation occured.
In this case, the adverb clause explained that Fran read several books a day while he was sick. It gave us an explanation on when Fran read several books.
