Exit Criteria or Iteration Backlog?

Can anyone expand on the definition of Exit Criteria and how it is used in an Agile Project? The definition from Process Guidance (below) is a bit confusing and I don't quite understand the value.

Thanks ! ~ !

" Exit Criteria is a Yes or No value indicating if the task is part of the iteration backlog. This field is used to synchronize the various views of the backlog (the scenario list, quality of service requirement list, and iteration plan). If the exit criteria field is set to Yes, the task displays in the project checklist query and work product. This field will be renamed to "iteration backlog" in a future release. "

[731 byte] By [claydevin] at [2007-12-21]