"Thumbnail" sketches are quick, abbreviated drawings, usually done very rapidly and with no corrections. You can use any medium, though pen or pencil is the most common. Thumbnail sketches usually are very small, often only an inch or two high. They're an important and fun part of the creative process. We tend to plan by visualizing! So, unless you have some other planning processes that work for you – thumbnails will quickly become your best friends and guides towards a better-executed 
art piece!