Volumes/MobileBackups

I use OmniSweeper to keep track of my memory/storage usage etc. And I've realised the "Volumes/MobileBackups" hard drive section is approx 248.1 GB in size, and this Retina Mac Pro I've had for over a year, and I'm still learning about Macs. I just wanted to be enlightened about what "Volumes/MobileBackups" is?