A sprint, which is a time-bound period of work, always starts with a sprint planning meeting. During the meeting, team members choose items from the backlog to work on during the sprint. The backlog is a list of requirements (both functional and nonfunctional) for the project. The product owner prioritizes the backlog items based on business need, risk, and value to the organization. The Scrum team members break down large requirements into manageable portions of work that can be completed in a sprint.

Reference http://smallbusiness.chron.com/project-management-cpi-numbers-mean-35541.html