The picture plane is a great tool for those learning how to draw. It helps the beginner understand how to look at a 3D object and convert that object into a 2D drawing. Once you try the picture plane, it almost feels like you've unlocked a cheat code for drawing. Things like foreshortened views that seemed difficult to impossible before now seem much more manageable. It's difficult to describe in words how the picture plane works, but once you have one made, its use is fairly intuitive. Luckily, it's easy to make a picture plane. The companion workbook to Drawing on the Right Side of the Brain comes with a piece of plastic that's designed to be turned into a picture plane. But if you don't have the book, you can use any piece of transparent plastic or glass that is the correct size. The only downside to doing that is that you will have to draw your own permanent crosshairs onto the picture plane--the piece of plastic that comes along with the workbook has pre-drawn crosshairs.
I have several videos that make extensive use of the picture plane, so please take a look if you are curious.