Graining
Graining is when the tyre surface tears and rolls into small ridges, cutting grip until the rubber wears back through it.
It happens when a tyre is sliding while too cold to work properly — common early in a stint, on a cold track, or when a driver pushes before the rubber has reached its window.
It can clear itself: drive through it patiently and the surface wears back to clean rubber and the pace returns. Which makes it a test of discipline, since pushing harder to make up the lost time is exactly what prolongs it.