Many lenses become physically longer or shorter when you adjust focus. In other words they have two nested tubes which telescope in and out as you rotate the focus ring. This design, while inexpensive to make, is generally undesirable because when a lens extends or contracts there will always be air and dust sucked inside it. And inevitably this yields a bit of dust buildup over years of use.

Many Canon lenses use rear focussing (RF) or internal focussing (IF) instead. In rear focussing the lens elements closest to the camera move back and forth when you focus but the frontmost elements do not. In internal focussing some glass elements inside the lens move within the lens barrel. In both cases the lens does not change length at all because all lens element motion is contained within the lens barrel.

Another advantage of rear and internal focussing is that the end of the lens does not rotate during focussing. Many lenses which extend when you focus also rotate, which can be a nuisance if you use polarizing filters or graduated neutral density filters.