Raking and Scarifying is the process of removing thatch and dead matter from your lawn. This is done by cutting from a vertical direction or pulling horizontally, which leaves the grass intact but removes material around it.
Thatch is a layer of living and broken down organic matter that sits amongst the lower level of your lawn. This is what gives your lawn a spongy feel when it builds up. The thatch layer helps to maintain moisture within the lawn especially during dry periods, so it does have its benefits. However, when it grows too large, it can suffocate and hinder the growth of your lawn, so you need to regulate this.