how is the first act of the play an exposition?

Act one focuses on exposition, we meet the characters and discover the relationships they have with each other. The initial theme deals with the relationship between Mr. and Mrs. Hardcastle. Mrs. Hardcastle wants to indulge in the city's culture, to rub off their country traits and live a little. Mr. Hardcastle, however, doesn't want the same things. He sees the French culture as being superficial and has no desire to take part in what he sees as frivolity.