What is Sprint Planning?
Sprint planning opens each sprint: the team answers why (craft a sprint goal), what (select backlog items the team forecasts it can finish), and how (break them into a working plan). The product owner brings an ordered backlog; the developers decide how much fits — capacity is the team's call, never imposed.
Timebox: up to 8 hours for a one-month sprint, proportionally less for shorter ones.
Worked example
A payments team plans a two-week sprint: goal — "merchants can issue partial refunds." They pull the top five backlog items (23 points against a 25-point average velocity), decline the sixth despite the PO's hopeful look, and split the refund API into six tasks on the board. Ninety minutes, one goal, everyone knows what Tuesday looks like.