In short, neck tension is how tight the bullet is held in the neck of the case. Too loose and the bullet could move or fall out during normal operations. Too tight and it will effect accuracy and possible cause case problems.
When neck sizing or FL resizing, the die reshapes the neck, usually but not always, stretching it, thus the need to trim the case.
Here is an article that will explain it better than I can.
http://www.accurateshooter.com/technical-articles/reloading/neck-tension-not-just-bushing-size/
