URL Trackers are used to assign credit for referrals. HubPages shares 10% of page views for life when a new user registers using your tracker.
They share 9% or 12% of traffic for 2 hours when you use it in links promoting your hubs, or others peoples hubs. So if a user first views your hub, then comes to mine you would get 9%-12% of my hubs page views for the next 2 hours. I don't meant to imply you get anything extra from your own hub, but you never know if they will register or go and view other peoples hubs after viewing your own hub, which you would get something from.
Everyone gets a default tracker assigned to them when they join here. They are rather ugly looking and obviously a tracker.
Most users prefer to create a custom tracker. Click "Create A New Tracker" Where it says "Tracking Token" is where you enter a new tracking code. Then enter a description so you can assign personal meaning to it. "Used on Example.com". Don't go crazy making custom trackers, you only get to make a total of 20 of these.
I think a user name makes a good tracker. You can add a tracker when linking to any part of hubpages.
If I was linking to the home page it would look like this: hubpages.com/_MyWebs/ . The URL for this page is: hubpages.com/question/41381/what-is-a-default-tracker
I like using my user name because thats what it looks like in a link, not a tracking code which some users don't like for their own reasons.
To refer users to sign up there are two URLs that are good to promote. The signup page and the tour page.
Signup page before: https://hubpages.com/user/new
Tour page: http://hubpages.com/tour/hubpages
I HAD to break my links cause trackers are not allowed in questions or answers. LOL
When using a custom tracker in a URL you MUST add a underscore in front of it and a forward slash afterwards or it won't work. _MyWebs/
Any URL with a tracker in it is going to redirect, the technical term is called a 301 redirect, the users to the normal URL without the tracker in it, so they get to where they was going and you get the credit for it at certain times.
I hope my answer makes sense. I just crawled out of bed and started answering this question. lol
Darkside has written a hub that goes into more detail and I suggest reading it to better understand how trackers work.