Atonement seems like a simple title. And… you know, it really is a simple title. Everything about the novel is a puzzle box, but the title means pretty much what it says. The book is about Briony's effort to atone for the really horrible thing she did when she was a kid and accused her sister's lover of rape.
Okay, okay—there might be another twist. The novel is not just about atonement, but is the atonement itself. Robbie tells Briony to write him a long letter explaining why she did what she did, and the novel is that letter. It's the task he set her. But by the end we know that Robbie didn't tell Briony anything; he died in the war. Briony gave him a happy ending—so the atonement, then, is the effort to remake the past and fix what she did in imagination, even if she couldn't in real life.
Still. Compared to point of view, the title really isn't trying to trick you. Maybe McEwan wasn't feeling as spunky as usual when he wrote it.