The Agile tab allows you to select whether to Enable Agile Features on the project that you are currently editing, by clicking on the "Enable Agile Features" box. Once the box is checked you will see these additional options:
Add Work To Backlog enables that all Tasks, Issues and To-Dos created on or associated with the project will be automatically added to the project's backlog.
Add User Stories to Backlog enables user stories to backlog.
Default Iteration Days Span allows the user to set a default day span for Iterations in the Task based on Start date, otherwise user should specify.