“Follies” is one of Stephen Sondheim’s most celebrated musicals and its revival at the Kennedy Center explains why. Not only does it contain some of Sondheim’s finest music and lyrics, but in its subtly complicated book by James Goldman, it tackles a fascinating theme: how people remember the past and relate it to their present lives. “Follies” takes place in a decrepit theatre where the “Weismann Follies” used to be performed. It’s 1971 and impresario Dimitri Weismann (David Sabin) has gathered his former stars for a party on the eve of the theater’s demolition. Derek McLane’s set exposes a huge theater stripped to its brick walls.