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".
