Rollers are an indoor training aid where you can ride your bike while standing still. Subset pf [trainer]
Contrast with turbo where you have to remove the rear wheel to fit a turbo trainer.
Rollers let you use your normal bike inside, without making any change at all. Your rear wheel sits on two rollers, and the front wheel may sit on its own single roller, or it might be held in a fork holder.
Rollers often fold in the middle for storage.
They frequently have some way of turning the front wheel too, unlike a turbo trainer where the front wheel is still. This enables the use of bike computers/cyclecomputers that use a front-wheel magnet, and mostly allow you to ride more naturally because steering is an important part of balancing.