This meeting takes place at the beginning of every Sprint. The Product Owner presents to the team the current vision and highest priority items from the the Product Backlog. After this presentation the team discuss and ask questions about the items so that they fully understand what it means to complete them. At which point the team forecast what they can deliver in the Sprint.
The inputs to the Sprint planning meeting include the Product backlog as described above and if the team has completed earlier Sprints then the velocity from these can be used to help guide the forecast for the Sprint.
During the Sprint planning the the team should consider a Sprint Goal. Agreed and created by the team, this provides focus throughout the Sprint and a metric to which measure at the Sprint review meeting at the end of the Sprint.
Suggested time box: Maximum 10% of Sprint
See our Game of two halves blog for an idea on how to run the planning meeting.