It's also known as aeration, and a lawn company will bring a machine to run over your lawn that takes out little segments of the soil to let more air down to the roots if your lawn is overly "thatched". That means it has a ton of dead stuff down by the roots preventing air, water and nutrients from penetrating the sod very well. The machine leaves little "cores" of sod laying on top of the grass so it looks like a whole pack of dogs were out there . . .
In some places with nice, loose soil it's really not necessary, but if you have heavy clay soil and you don't collect your grass clippings, but let them mulch, it's not a bad idea to do it every few years.
Depending upon where you are, this isn't necessarily the greatest time of year to have this done. Springtime is better when the grass will grow back quickly. Would you fill in your profile with your location, please? This will help us to answer your questions better in future.
