Blade tracking is when both blades are flying in the same plane, from the side you will see one straight line. OOut of track bladese appear as 2 blades side on, this causes vibration.

Normally if you set the blades to the same pitch with a pitch gauge and they are good blades they will be in track, if they are cheap blades or out of balance the first thing you do is carefully balance them, then you can adjust the links to the blades to get them to match up.

There's some pictures and such on http://www.raptortechnique.com look around at his pull down links, one of them describes it.