From the Super Mario Wiki
Rollers are stone-like obstacles that appear in Mario Kart 8 only in the track Thwomp Ruins. These obstacles roll around certain parts of the track after 45 seconds in a race. At one point, a roller will roll over a wall blocking a glider panel, thus making it available for use. Unlike most hazards in Mario Kart 8, slowly driving into a roller will not make the racer tumble over.