By default, you view and edit your presentations in Filmstrip view. This presents the timeline of the presentation as thumbnails in the left column, while letting you edit the selected slide in the main part of the window.
The view changes, and the selected slide no longer appears in editing mode. Instead, you see a grid of thumbnails representing every slide in the presentation.
Since the entire screen is taken up by slide thumbnails in Grid view, you need to return to Filmstrip view in order to edit a slide's contents. You can return to Filmstrip view the same way you switched to Grid view, but if you want to edit a particular slide, you can jump right to editing that slide quickly.