Screenshot: the in 'Plan' mode (click to enlarge)

In Plan mode you can:

• Prioritise the Backlog — Create issues for your backlog, rank and estimate them, and pull the marker (the grey 'Sprint' box) down to grab a set of issues you'd like to tackle for your first iteration.
• Estimate Stories — You can use the 'J' and 'K' keys to move through issues in the backlog and get details on the right-hand side of the screen. Plug in your estimates or story points as you go. (Note that, by default, the Story Points field is only available to issues of type 'Story' or 'Epic' — you can change this as described in GreenHopper Custom Fields and Database Changes.)
• Create Sub-Tasks — To break a story down into implementable chunks, go to the sub-task tab (click the folder icon) to view and create sub-tasks.
• Plan, and Plan Again — When you're happy with the stories for the iteration, click the 'Start Sprint' button and the stories will move into Work mode. While a sprint is active in Work mode, you can still plan a subsequent iteration in Plan mode, but you won't be able to start it until the active iteration is completed. (You can, however, drag and drop an issue in Plan mode onto the active sprint.) Note that you can only start (or complete) a sprint if you have 'Project Administrator'  for all issues in the sprint.

An issue will only be visible in Plan mode if:

• the issue matches the 's Saved Filter (see Configuring Filters); and
• the issue's status maps to one of the 's columns (but not the 'Done' column); and
• the issue is not in an active sprint.

Note that Plan mode is currently in Labs. Also note that Plan mode is only available for s of type Scrum — see Creating a Rapid Board.