This lesson is all about Primary, Secondary and Tertiary shapes, what they are and how to use them. This concept is also sometimes referred to as "Big, Medium, Small", as in, if your image has a nice distribution of big (primary), medium (secondary) and small (tertiary) shapes, the resulting image will tend to be more pleasing to the eye.
First, a definition. Primary shapes are your big shapes. If you squint at an image, the details tend to disappear and you're left with only your big shapes.
Secondary shapes are the smaller shapes that either sit ontop of, or help make up the primary shapes.
Tertiary shapes are again smaller than the secondary shapes.
First off, what should the size of these details be? Well, that depends on personal taste. I like having the primary details being huge, almost the size of the image itself. And I like Tertiary details being really small, sometimes the size of a pixel or two. From there you can decide the appropriate size for the Secondary Details. Of course, not all details should be the same size, it's more like a range of sizes, so not all secondary details should be the same size, but should be in a similar range.
So these could be primary, secondary and tertiary shapes....