They're just a set a bellows with a mount on each end, on a rail to allow precise setting of the distance between the mounts.
Exactly the same principal as extension tubes, but you can control the magnification.
I wouldn't recommend it for OP though, bellows are more specialised for tripod work and have no electrical contacts, so require aperture to be set on the lens, something which is not possible on native EF mount lenses (without using the dismount while holding DoF preview technique).
Extension tubes are less flexible, but much easier to work with as you can get them with electrical contacts.