If you choose to autotrain a skill, you don't spend points in it as you level up. Every few character levels it will automatically raise your level in that skill (this will give you additional points leftover to spend elsewhere). Take note that you can't autotrain in everything, each class has specific speclines that can be autotrained in. While autotraining will put you at a slight disadvantage while trying to level up to 50 (because there will be times when you aren't spending the points you can spend in favor of waiting for that spec to autotrain), in the end run it will give you a few more points to spend. At least that is how I understand it.