Sulcal effacement is a local secondary sign of mass effect in the cranium. Any lesion exerting mass effect on brain parenchyma can push adjacent gyri together, thereby displacing the CSF from the sulci.
It is greatest at the site where the cause of the mass effect is located (e.g. tumour, haemorrhage, abscess, etc), a useful sign when the lesion is isodense to adjacent brain parenchyma and therefore easily missed.
For a general discussion please refer to the definitional article: efface