Slides often have objects overlaying each other. For example, you might want to show an Autoshape or text over the top of a picture. You need to be able to specify the order of the objects so that the right one appears on top.
Click the Draw menu (or right-click) and select Order. Choose from:
Bring to front – bring the selected object to the front of the heap
Send to back – send selected object to the back
Bring forward – bring forward one place
Send backward – send backward one place
Note that if you want to have the same background to all your slides (for example a picture with some text underneath) use the Slide Master rather than trying to duplicate it on every page.