It's a built in feature on a Mac that lets you do a clean reinstall of the OS.
Restart your Mac, as soon as it turns on hold down Option-Command-R, and release the keys when you see the Apple logo.
Once it loads you'll see a OS X Utility window.
From the options in the OS X Utility select Reinstall OS X.
Follow the on screen prompts, you'll be assed to connect online to get the newest OS, and after the reinstall and a reboot, you can setup all the information such as time zone, language, username, etc.