"Tracking someone down" means finding out where they are. For example, you might "track down" a coworker who you need to talk to, or you can "track down" an old friend from grade school that you haven't seen in many years.
Grammatically, the phrase can be "track ___ down" or "track down ___". Pronouns usually come between "track" and "down":
We tracked them down.
How'd you track me down?
Longer words and phrases usually come after:
You should try to track down the person who sold it to you.