Agile Overview → Manage Demand
A work item is added to the product backlog by product owners and team members. A work item is moved into a release backlog (from the product backlog) to support the goals outlined in the product roadmap. A work item in a product backlog should be estimated (at a very high-level), broken down into a smaller work item (with some detail), and so on, before the beginning of the next release.
Product planning meetings may include team members, but they often include high-level stakeholders who are not team members, such as executive sponsors, members of other teams within your organization (support, documentation, marketing, and so on), as needed. The goal of product planning is to make sure that your product roadmap contains only a work item with the highest business value and priority for the upcoming 2-4 releases.