The Slide Sorter pane allows you to select individual slides, change the order of the slides and insert and delete slides.
To select a slide, click on it in the Slide Sorter pane.
To insert a new slide, click on the where you want the new slide to be inserted and click on the Insert New Slide button (or select New Slide from the Insert menu). You will be prompted to select a layout for the new slide.
To delete a slide, click on it and select the Delete Slide button (or select Delete Slide from the Edit menu).
To change the order of the slide, click on it in the Slide Sorter pane and use the Move Slide Up and Move Slide Down buttons.