Some lenses have an outer end which rotates when you adjust either the focus or the zoom setting or both. Others do not.

This matters a lot if you’re using a polarizing filter or a graduated neutral density (ND) filter, since the properties of the polarizer vary depending on its angle of rotation and the graduation line runs across the filter in a straight line. It can be very annoying to set a filter to achieve the effect you want, touch up the focus, and find that the polarizing or graduated ND effect has changed because the end of the lens has rotated.