You spread sand on your lawn, and it levels out (eventually) the base of your grass. Let me find a youtube video.
Edit: Here's one. There are literally hundreds of videos on youtube. I did it to the front yard of my farm in Maimon and my house in Arroyo Hondo:
No need to go crazy like this guy, You can use a bucket and shovel.
Just think about how flat sand is at the beach. Same concept. Just don't use beach sand.