Tracking is the practice of being able to discern the current location of a moving target, and even follow if over long distances.
Skilled trackers are often able to notice traces and evidence of where their designated targets have gone, and where they will be next, after following the trail for some time.
In the Hunger Games, because survival is the primary goal and the only way for that to happen would be if tributes eliminate each other until only the victor(s) emerge, many tributes chose to avoid confrontations and distance themselves away from the other competitors. Because of this, trackers are usually a weakness to these tributes that chooses to avoid confrontation, as their location might be found.
Also, during the 74th Hunger Games, the tracker is a way to tell whether a tribute is alive or not. The tracker is placed in a tributes arm and glows a bit after the tracker is placed.
Advantages and Disadvantages
The advantages with this skill is that it's easier for a stronger tribute to find other tributes to prey on, where as a tribute without this skill might find it difficult to find new victims without the gamemaker's interference. However, the tactic itself is not absolute, since trackers have been known to get lured into traps and ambushes after trailing what they thought was unwary prey.
One of the known users is Rue, who tracked and followed Katniss before they formed an alliance