anti-roll bar = sway bar
It is that relatively thick bar that connects to the suspension rig at both wheels and then juts out behind it. That's for the rear. In other words, look underneath your car from behind and you should see a black bar going across hanging down a bit. There is one on the front too, it is forward of the wheels and has a bit more bracing. It is what the oil drips down on when you remove the front drain plug.
Basically, a thicker bar reduces body roll and helps you have a firmer suspension. Most companies also produce the bars to be as strong as stock, but thicker and lighter weight. That way not only do you reduce body roll, you lighten your load.
Look for adjustable bars - generally has sets of bolt holes that let you pick the firmness, depending on the type of road conditions you are racing on.