Thumbnail sketching is basically doing very small, simple sketches. Usually just with line and maybe some rough hatching, but they can be shaded, or in simple blocked tone. A thumbnail sketch is usually enclosed within a rough frame, so that you are laying out a small but complete composition. This is a great way to experiment with various compositions or arrangements. Its also a good way to ensure that you consider the entire picture - too often we tend to 'float' the central object of interest in the middle of the page. Click the link to learn more about thumbnail sketching.