The Dependencies page shows the dependencies that your plug-in has on other plug-ins. The plug-ins your plug-in currently depends on are shown in the list. You can modify the list by adding more plug-ins (button Add...) or removing them from the list. Changes in this list will prompt PDE to update the plug-in manifest and also configure the build path of the project so that the classes of the imported plug-in are visible to the plug-in under development.
Since the PDE distinguishes between workspace and external plug-ins, the eligible plug-ins will be shown in two separate lists of checkboxes.
Once you add the plug-in reference to the list, you can browse it by selecting Open from the pop-up menu or double-clicking on the entry.
When you select an entry in the list of required plug-ins, you can re-export the dependency in the Import Details section. Re-exporting will make the plug-in visible to plug-ins that import it. You can also be more specific regarding the version and matching of the requirement. By default, any plug-in that has the required ID will work. You can add constraints by specifying the version and whether the version match should be explicitly specified (you can read more about valid values in the Core ISV guide).
The section in the bottom-right corner can be used to obtain dependency graph information. You can select the type of information from the drop-down combo box. The section can be used to check loops in the dependency graph, fragments that reference your plug-in and other plug-ins that require your plug-in. In each case, the result will be shown in the list below the combo box.
By default, changes in the dependencies list will automatically trigger updating of your plug-in project's build path. You can turn that feature off on the Plug-in Development->Java Build Path Control preference page. When this feature is off, if you add or remove a plug-in in your dependency list, you are responsible for updating the build path so that the required entries are added. If you do not, the Java compiler will not be able to resolve references to types that belong to the required plug-ins. See Build Path Computation for more details.