This means that you cannot for whatever reason establish a connection to the Tracker needed to download a file.
This is commonly caused by firewalls/routers, trackers/servers going down or being too busy, or your net connection going down.
You may also be banned from using this tracker either for leeching from it.
More often than not with this error you can simply leave your client window open and it will sort itself out, or you can close the BT Client and try resuming the torrent.
If the problem is persisting make sure the Tracker you wish to connect is operating, and make sure your firewall/router is not interfering with TCP traffic on the BT ports.
If after a few days your download still does not start or continue and you have correctly configured your environment, then you can safely assume the tracker is gone.