A Pull system is a production method that creates product to match customer orders.
Imagine if you walked into a Chipotle (or Subway, Quizno's, Potbelly's, or any other quick-custom restaurant) and ordered your burrito, and you were told to hang on while they looked through the pile of already-made food for one that matched your order, if they could find one with all of your special requests. No one wants to eat an hours-old burrito.
Or, what if you ordered your burrito and then had to wait an hour while all of the ingredients were prepared? Fully customizable, you could opt out of lime in your rice, but your lunch hour is over before you eat.
Restaurants like Chipotle are working to strike the right balance between customizable (white or brown rice?) and ready to go (sorry, the cilantro is already in there). As soon as a customer walks in the door, the restaurant springs into action to fill the order. While that's happening, as well as before and after, other members of the team are working to resupply the assembly line with prepped ingredients like chopped vegetables so they don't run out.