Tracking allows you to view a list of nearby humans, monsters, or animals.
From Telamon: Tracking on Renaissance is the Trackers Tracking + Detect Hidden skill pitted against the Targets Hiding and Stealth Skills. So to track someone with no hiding or stealth with 21 tracking, would show you the target within a range of 21 tiles.
Raising Tracking: Tracking is an Activated Skill on the standard 10 second cooldown. Tracking is a difficulty based skill - higher tracking allows for a greater range. Use Tracking repeatedly until you reach Grandmaster.
Gideon's Advice: Tracking is useful even at lower levels - a lot of builds put any extra points available into tracking. This has been changed recently so GM is much more useful but lower levels are still handy.