Tracking is how well a boat goes straight and holds its course. A boat that doesn't track well will want to veer off of the path you are paddling, requiring constant correction strokes. Obviously, this is somewhat user/skill related because the best paddlers can always get the boat where they want it to be, but good tracking helps. A lot of things affect tracking. Flat hulls, playboats, want to spin and don't usually track as well. Long, round boats will slice through the water and tend to track well. A paddlers weight relative to the boat can have a major affect on tracking. A paddler that is heavey for a boat will cause the boat to sit lower in the water which will usually make the boat really good at going straight but more difficult to turn and maneuver. A paddler that is too light for a boat will often end up sitting too high in the water and this will put less of the boat in the water to help tracking (think about how easy it is to spin something that sits on the ground at a single point like a ball verses the difficulty in turning something with a large contact area). You can see how rocker can affect tracking in this way, too. With flat-hulled boats that want to spin, you can put the boat up on edge to help its tracking.
That website is pretty sweet. I can't do any better than they do on most of those terms.
"The world would be a better place if everyone kayaked."-Brad Ludden (Valhalla)
"You only get one chance to run a drop blind."-DD