These rails are mounted on the sides or beneath the drawer box to reduce friction when opening or closing the compartment.
Drawer slides not only come with different mountings: undermounted or side mounted but there are different types of slides depending on the material. Wooden slides are the simplest, constructed with horizonal rails that the drawer box sits on. Roller slides have metal brackets and wheels and slide along each other when the drawer opens and closes. Ball bearing slides use ball bearings for smooth glide movement.