Scarification is for removal of excess thatch in your lawn OR when you have a lot of dead moss to extract.

Thatch is a layer of dead material in between the soil and your grass. You can tell when there is an excess layer through visual inspection (lift a bit of your turf up) and when the grass feels spongey to walk on. A small layer of thatch is fine and can go untreated.

Removing excess thatch, like aerating your lawn, will allow your grass to access air, water and nutrients more freely and therefore improve overall health and growth levels.

You can scarify by simply raking with a spring tine rake or using a mechanical scarifier. A mechanical scarifier is quicker and more effective and can be rented for the day from places like Hire Station. When you're scarifying make sure to do 2 passes of your lawn in different directions for maximum effectiveness.