Tracked Fields are a special type of custom field that collects data about a person on an individual team. They are team-specific fields, and each value can be different per team.
Think about attendance tracking: a person can attend some teams, but not all. A profile-level field would not suffice to track attendance because the value is going to depend on the team.
Another example that tracked fields can be used is when you want to track team output per person. Like, "How many items did someone author on a team?" or "did someone complete all of their given tasks on a team"?