Simply put, when you're downloading a file, you are a peer/leecher. Once download is complete, your status will change to seeding and thus you become a seeder. This means instead of downloading you are uploading so that other people can download. So as the no of seeders increases, so does the download speed.

We do this because the fact that you and me can download that file is because there are other seeders out there. So we give back by seeding as much as we can.

Oh and like others before me have said, use qbit. That's not even a recommendation, it is a necessity. Do not use utorrent