Dethatching (AKA power raking) loosens and removes thatch that accumulates naturally over time. 

Lawns can become compacted by grass clippings and debris that builds up. When the thatch layer becomes too thick, 

it can block water and nutrients from properly being absorbed.  Dethatching can help remove the sluff and freshen up old, hard grass areas. 

*RECOMMENDATION: For best results, overseed after dethatching.