Jamming or Jumping a bullet, when it comes to reloading techniques, has to do with the seating depth of the bullet when loading and how far the bullet will be from the rifling of your barrel when chambered. Seating depths are often referenced as either jump or jam distances. Jump is the distance away from the lands of the rifling and Jam is distance into the rifling that you are forcing the bullet when you chamber a round by closing your bolt.
When jumping a bullet to x, that will be the distance the bullet is seated so that when the round is chambered, the surface is x distance away from the lands. E.G. A bullet that is jumping 0.015″ would measure as that distance from the rifling.
When jamming a bullet to x, that will be the distance the bullet is seated so that when the round is chambered, the surface goes into the lands and ends up being pushed back by x. E.G. a bullet that is jamming 0.005″ is being forced into the lands by 0.005″ when you close the bolt of your rifle.
For more detailed information, please read the Handloading Basics chapter of the Berger Bullets Reloading Manual.