What does the title Silver Linings Playbook mean?

Football is incredibly important to the main character, Pat, and his family. In that context, a playbook is "a compilation of strategies the team would like to use during games". This usage is reinforced by the playbook graphics on the film's promotional materials.

The poster uses quotes from each character as explanations for their "plays", which I would say is equivalent to their outlook on life and strategies. At the beginning of the movie, Pat believes that if he works hard and improves himself, everything he wants to have happen will happen. He thinks that every bad thing will end up having a positive effect, leading him to his goal of reuniting with his wife as a better husband than before. The movie is primarily about him figuring out the strengths and failings of his absurdly optimistic approach, or "Silver Linings Playbook".

John Milton coined the phrase 'silver lining' in Comus: A Mask Presented at Ludlow Castle (1634) in reference to clouds. Since then the phrase has represented a prospect of hope or comfort in a gloomy (cloudy) situation.

To the less literary-inclined, the Grateful Dead had the song, "Touch of Grey" that's central theme was a silver lining.

A playbook suggests a means to attain that "silver lining."

Quoting from Wiktionary,

every cloud has a silver lining, proverb

In every bad situation there is an element of good.

And from Urban Dictionary:

phrase used to tell someone that there is a brighter side to the problem they are facing. the phrase comes from the fact that, every dark rain cloud has a silver edge, or lining.

So as other users commented, the silver lining describes a good side of something that is apparently bad. In the movie's plot, we can find that the main character Pat (Bradley Cooper) is having a bad time but when he meets Tiffany (Jennifer Lawrence) the situation is still bad but with a better approach.

From the Wikipedia page of the Novel-

Pat believes he has only been away a few months, but soon realizes it has been years, and struggles to piece together his lost memories and what has become of his wife, Nikki. He has a theory that life is a film created by God and that its "silver lining" will be the end of 'Apart Time' with Nikki.

So this line shows the real intent of naming the novel this way and the movie just copied the title.