Marbles are pieces of shed tire rubber and loose track material that collect outside the racing line and are slippery like toy marbles. Drive through marbles at speed and the car gives up grip instantly.
The published definition is an asphalt one. Wikipedia’s motorsport glossary calls marbles “pieces of rubber from tires that accumulate on the racing surface outside of the racing line, that are slippery like toy marbles.” On dirt the same word covers different material — loose clay and dirt balls rather than shed rubber — and the consequence is identical. The dirt wrinkle is that the loose stuff pushed to the outside is the cushion, and dirt racers lean on it deliberately, so on dirt “marbles” usually means the junk that ends up where nobody is running rather than the berm at the top. Either surface, the practical problem is the same: a car defending the line forces you off it, and one lap through the marbles hands the position back.
Marbles are why a pass that looks available off the racing line usually isn't, since one trip through them costs more time than the pass would have gained.