A user who has downloaded a torrent and is letting others download it from them as well is a seed.

To become a seeder, keep your torrent client running after completing a download and do not stop/remove the torrent. Also perform actions like rename/move from within the client so that the files remain accessible.
When someone downloads data from you, you will get upload data added to your account.

To avoid getting an H&R flag you have to seed torrents and complete the minimum seed time requirement.