Scarification is the act of partially removing the thatch layer (and moss if it is present)
Lawns create the thatch layer through the normal living growing and dying processes. The thatch layer is unable to break down itself. The material has the wrong carbon to nitrogen ratio (Too much carbon not enough nitrogen) much like the principle that allows the other “thatch” on the roofs of houses to last 25 years. Thatch layers below 0.5” can benefit from lighter treatments, anything deeper than this you will need to work hard on.
This thatch layer causes the following problems:
Store for disease
Prevents air water and nutrients from reaching roots
Ideal home for some insects
Chokes grass growth, so reducing shoot density of grass
Partial removal is carried out with a scarifier. If your area is small you can use a spring tine rake, but to be honest this cannot remove sufficient material.
The tool you should be looking to hire is a vertical knife type scarifier. This has blades that physically cut the thatch out.
You should proceed at a shallow depth to start with across the whole lawn.
Then go across the lawn again at a different angle and slightly deeper.
2 or 3 progressive cuts are better than 1 deep one.
Aim to still leave a green lawn behind. These machines are powerful enough to remove a lawn so be careful. There should be at least 75% of your original lawn left.
Remove the debris to your compost heap making sure it is well rotted before ever bringing back to your lawn.