When you place an image on your page (for example go to shape tools and choose Graph. Rect.) and you look at the properties panel you can see there the option Stencil just below the Margin property and above the Text property.
Click the Stencil property and a new window will open. At the top you can choose your own made stencil (you can make a shape in any graphical program but make sure the background is transparant and you save the image as a transparant png file).
In the centre you see an example of the choosen stencil.
At the bottom you can choose from a list of pre-made stencils.
These stencils are made to help you easily create certain shapes. Just to make live a bit easier.
The same goes for objects to just make live a bit easier but now you don't need to first create an image on your page. The objects are ready to be inserted just the way they are.