The idea that the minute you start a new job, or a new task, you can immediately make an intelligent and significant contribution, despite not having a clue what the hell is going on.
Most often encountered in references to new hires, recruiters and managers often like to use phrases like this: "We're excited for you to join the team Sammy. You're going to have to dive right in, and hit the ground running on that big Balls in the Air project. We're counting on you to pull through on this!"
It often translates, loosely, if you were chatting with friends outside of work, to something like this: "Sammy, we don't have a damn clue what the hell we're doing with big Balls in the Air, and neither do you, so, since we don't have budget or processes in place to bring you up to speed, you get this beast which lets us veterans watch you sink or swim and gives us a scapegoat while also freeing up our schedules to leave work early. Thanks!"