Cycle Rollers are a very basic, and old fashioned, version of the modern turbo trainer. Basically 3 rollers, 1 for your front wheel, and 2 for the rear - they are connected by a rubber band so they roll in unison.
The difficult thing is mastering how to ride on cycle rollers as the bike is liable to fall off the side. Modern rollers have a tapered front wheel roller that helps self position in the middle. A top tip is to position the rollers next to a chair so if the worst happens to can grab the chair to stabilise yourself.
It has been said that they are harder to ride, make you more tired and you are liable to fall off more than a static turbo trainer. It is also harder to stand up so can lead to numbness. Many road racer use them for short term warming up before races, you will see them at a Velodrome as they are portable and do not touch the bike (i.e. no clamps).