thatch : noun meaning “a matted layer of dead stalks, moss, and other material in a lawn” (Thank you Oxford Dictionary)
In lawn care, thatch is a layer of organic matter that accumulates on a lawn around the base of the grass plants. Thatch is a combination of living and dead plant matter including crowns, stolons, rhizomes, and roots. (Thank you Wikipedia)
Here is a YouTube link to some lawn care professionals talking about thatch and the process of removing thatch with a lawn dethatcher.