Basically, it's a feature that lets you create your own customized jobs and then share them with other players. It's currently in beta, and there are two types of jobs available: Deathmatch and Racing. You create a title, description, and - most importantly! - the job itself, covering details like how many players you want, time limits, score limits, weapons, trigger locations, spawn points, and more.
Check out the video from Gamespot below for a more in-depth explanation, and let us know in the comment section if you've cooked up any custom jobs already: