Tracker has several meanings:

1. A person who follows the signs left behind by animals or people to find them. Native American trackers could read footprints, broken twigs, and disturbed leaves to follow deer through a forest or locate someone who was lost. A skilled tracker notices tiny details others would miss: which way grass bends, how deep a footprint sinks, whether dirt has been recently disturbed. Modern search and rescue teams still use tracking skills to find lost hikers.
2. A device that monitors where something is or what it's doing. A fitness tracker counts your steps and measures your heart rate. A package tracker shows where your delivery is on its route to your house. Some parents use GPS trackers to help keep their children safe.
3. A tool for organizing information, like a habit tracker where you mark off each day you practice piano, or a reading tracker where you record books you've finished. Teachers might use a grade tracker to monitor student progress throughout the semester.

The core idea connects all these meanings: following or monitoring something over time to know where it is, where it's been, or what it's doing.