Seeding is simply you're computer making the file available to others. You downloaded your torrent file from other people who were seeding, and as long as people are seeding the file is still available.

As for why its taking so long, technically it goes on indefinitely, as you can always share you're file. If however you feel its taking up too much bandwidth you can always stop seeding, or adjust how much bandwidth you're willing to let the program use. If you adjust it, then you can keep seeding (more seeds = better download speeds for others) and still be able to use the internet or other programs without taking much of a performance hit.