RDS stands for Removable Deployment System.
It's a device designed to remove the drag caused by your slider, d-bag and pilot chute, physically detaching these items from your canopy after opening.
The RDS is essentially a special type of slider with hardware that allows you to remove it from above your head. The components can be broken down into 4 rings, that are installed on each individual riser.
A slider attaches to these via Teflon release cables, and these feed into the slider itself and allow the slider to be detached. Then there's a lanyard that comes off the back of the slider, which allows the regular d-bag and pilot chute to be connected.
After opening and with the brakes still set, the release cables are pulled and held onto in one hand, which allows you to detach the slider and wind everything into your body. After this it is typically stored in a pocket either on your pants or in a belly mounted pouch. Packing an RDS can be quite a complex process and requires careful instruction.
It's easy to misroute either the cables or the lanyard and cause a serious malfunction or entanglement.