A seed is a user who uploads files to the BitTorrent network for other users to download.
A leech is a user who downloads files from the BitTorrent network from other users.
A typical user starts as a leech by downloading a file. Once the file is finished downloading (or even before), the user transitions to being a seed and starts uploading the file to other users.
A common courtesy among torrenters is to seed as much data as you leech. So if you download a 1 GB file, you should seed that file until you’ve uploaded at least an equivalent amount of data. However, this guideline is in no way enforced.