When one has finished downloading porn/pr0n and leave the files available so other may download (upload) said porn from you. Usually done with bit torrent and helps to boost your uplaod download ratio on particular trackers. Most sites enforce a ratio policy so others can get what they want and not have leechers.
As you can watch porn while seeding porn not only are you sharing the love - you can still feel good afterwoods!
"I need to get my ratio back up to 1:1"
"Dude, seed your porn"
"I keep my ratio healthy by seeding porn"