The document discusses the concept of activity-centered design for mobile apps. Successful mobile apps focus on supporting a single, well-defined user activity like photo sharing on Instagram, maintaining social connections on Facebook, or sharing status updates on Twitter. Early social networks and apps like Burbn and iTunes Ping tried to support many different activities but were replaced by apps that focus on one core activity. The document advocates that mobile app design should center on supporting one specific user activity extremely well rather than trying to be good at many things.