To seed, you just leave the torrent program open so that other people can download from you. Bittorrent works by downloading different parts of a file from different people at the same time. Because upload speeds are generally a lot slower than download speeds, uploading is the bottleneck in transfering files. Bittorrent gets around that bottleneck by allowing you to download one part of a file from one person, and another part from another person at the same time. The more people who seed (allow others to download from them), the faster everyone's download will be.