Seeding means sharing a file(s) with other peers.  After a torrent job finishes downloading, if you leave the torrent job seeding, it uploads the file(s) to other peers so they can enjoy them too.  Although the length of time that you should leave the file seeding is not defined, it is recommended that you share until the amount of data you upload reaches at least the same as the amount of data that you have download, also known as reaching a 1.0 ratio. It’s difficult to know exactly when you hit this ratio but generally people who leave the swarm before even getting close to that ratio are labeled as leechers.  Because leechers have a detrimental effect on swarms, some peers or even private trackers may block connections to you.

source : BitTorent