A BitTorrent Tracker is a server that assists in the communication between peers using the BitTorrent protocol. Also, in the absence of extensions to the original protocol, it is the only major critical point, as clients are required to communicate with the tracker to initiate downloads. Clients that have already begun downloading also communicate with the tracker periodically to negotiate with newer peers and provide statistics; however, after the initial reception of peer data, peer communication can continue without a tracker.
A tracker should be differentiated from a BitTorrent index by the fact that it does not necessarily list files that are being tracked. A BitTorrent index is a list of .torrent files, usually including descriptions and other information. Trackers merely coordinate communication between peers attempting to download the payload of the torrents.
Many BitTorrent websites act as both tracker and index. Sites such as these publicize the tracker's URL and allow users to upload torrents to the index with the tracker's URL embedded in them, providing all the features necessary to initiate a download.
In order to seed a Torrent you need to make sure the files you want to seed are in the right folder. If they are and you've got peers connected to the Torrent, it will automatically start seeding if your settings are set by default.
Please make sure your PC is on, and your Torrent client is active for seeding. Once you switch it off the connection will be lost. If other users have not completed the download, everyone will be waiting for you to reconnect. You should consider this when posting new Torrents as it's up to you to seed until others have finished.