Tooth is the name for the microscopic texture of a paper's surface, including the tiny peaks and valleys that help colored pencil material adhere to the paper.
Think of watercolor paper and their various grades of pressing from Cold-Press (very toothy and textured) to Hot-Press (smoother with less tooth and texture). And markers often prefer polished or coated papers where the tooth has either been sanded away or pre-filled with a sprayed on layer of minerals or polymers.
For colored pencil, paper with a lot of tooth has a rougher surface which can hold more colored pencil material. Smooth paper has a finer tooth, which can create smoother color application but smoother paper accepts fewer layers of pencil.
As colored pencil layers accumulate, pigment and binder gradually fill the valleys of the paper's tooth. Heavy pressure and aggressive burnishing can also flatten or damage the surface texture, reducing its ability to accept additional color.
Aggressive burnishing can also fold the tooth down over previous layers of pencil, locking the color in place and creating strange patches of pale or matte color.
Once the tooth is filled, it can be very hard to add additional layers of pencil. This is when you’ll notice issues with adhesion, laydown, pilling, shedding, or trenching.