Marbles
Marbles are the small pellets of discarded tyre rubber that collect off the racing line, and they are extremely slippery to drive over.
They build up through a race as tyres shed rubber. Because they gather everywhere except the line the cars actually use, going off-line to overtake means driving on them — which is a large part of why passing gets harder as a race goes on.
Drivers deliberately run through them on the slowing-down lap: picking up rubber adds weight, and cars are weighed after the race.