Drawer runners, linear slides, telescopic ball bearing slides....whatever you wish to call them, they are all basically products that move things up and down, backwards and forwards, position things in the correct place and give access.
The ball bearings are controlled by a ball retainer. This holds the ball bearings in place & allows the members of the slide to move relative to each other. With precise design and manufacture the members and the retainers move at a ratio of 2:1.