Uninstalling a package means removing it from your system. This is useful to gain disk space. Of course, you can only remove already installed ones: so select a package in the Installed list. For example, let us try to uninstall the emacs package, which has been installed so emacs-pcomplete can work. Again, use the flat list to find it easily, then select it. You can also use the Find tool.
When this is done, click on the same button you clicked on to install a package. The RpmDrake window is replaced by another wizard, which tells you that (at least) another package will be removed, in our example emacs-pcomplete. This makes sense: we've been told earlier that emacs was necessary for emacs-pcomplete to work. If we remove emacs, then emacs-pcomplete won't work any more, so we should also remove it -- to keep the whole system consistent.
At this point, if you click the Next, the packages are removed from your system. Note that you can still install them again, if you wish to use them after all... Just click on Quit to get back to RpmDrake.