This might be an overkill explanation, but if anything hopefully it can help clear things up and get you started. As stated above, jump is how far away from the lands of the rifling you are. Generally the less jump you have the more accurate your load will be, as there is less "wobble" and unguided travel for the bullet to go from where it sits in the case to when it hits the actual lands (at the 'ogive' - the point in the curve of the bullet that becomes the actual diameter of the barrel). Most often, you need long, heavy, high-bc bullets to be able to seat out far enough, since a bullet of less weight will be too short to reach that point. Remember to always keep at least the diameter of the bullet seated into the neck, or close at least. Generally a good starting point is around ~20/1000 off the lands, +- 10. Increments of 5 are usually good. It's dangerous to seat the bullets right at the lands as that can lead to pressure problems (causes dangerous pressure spikes). The way most people do load development that way is to figure out charge weight that will get you the most accurate, and once you have found the best load then start playing around with seating depth. Again, that's just one way and I'm sure other people have their ways of doing it, that's just mine and what I've seen the most. Hope this helps!
