What is a TRACKER?
Heres an example:
A server on the Internet that acts to coordinate the action of BitTorrent clients.
When you open a torrent, your machine contacts the tracker and asks for a list of peers to contact.
Periodically throughout the transfer, your machine will check in with the tracker, telling it how much you've downloaded and uploaded, how much you have left before finishing, and the state you're in (starting, finished download, stopping.)
If a tracker is down and you try to open a torrent, you will be unable to connect. If a tracker goes down during a torrent (i.e., you have already connected at some point and are already talking to peers), you will be able to continue transferring with those peers, but no new peers will be able to contact you.
Often tracker errors are temporary, so the best thing to do is just wait and leave the client open to continue trying.
To sum up, think of a TRACKER as the 'switchboard' to get the files you need.
To find out what Tracker to use when say 'Making a Torrent', you may need to refer to the website you wish to post the torrent on.
Most good websites will have there TRACKERS URL on there somewhere.
If not, you can use Public Trackers, for a list - Click HERE