You can mask any graph you draw. Draw your graph and then push the mask button.
Any graphs you have drawn are removed from the axes and the axes are overlaid by a faint "mask"
The mask will not appear on any generated graphics. You will only see it on the screen as you are drawing. It is there so that you know that the graph has been masked.
Why?
You might use the mask to hide the answers to a question. You can draw up the answer but then mask it so that it is not displayed for students. Later, when creating solutions to the question, you can quickly unmask all of your work.