Scarification is the mechanical process of removing thatch and moss from your lawn.
Thatch is a combination of living and dead plant matter including crowns, stolons, rhizomes and grass roots. Grass clippings do not generally contribute to thatch buildup.
We generally scarify in the Spring, March-May and also in Autumn, September-October when conditions are ideal for recovery. Your lawn may need over seeding afterwards depending on the amount of thatch and moss removed.