A description of what trackers are and how they work is desperately needed. It's not at all intuitive.
In case you're still stumped, I played around and finally figured it out:
• Give the tracker a name, just like you would any tag
• Then when adding that tag/tracker to an entry, you're asked to fill in a value
Example:
• I make a tracker called "Weight"
• Each time I add that tracker, it asks for a value. I enter, for example, "185.4"
RESULT: The entry has a "tag" that reads "Weight 185.4"
UPSHOT: You can track your weight in tags.