First, what is seeding?
Seeding is a process where you keep your BitTorrent client open even after you've finished your download to help distribute the file to other users.
What is web seeding?
Web seeding is when a torrent client has the ability to download the torrent's data from an http source, like an online server, in addition to the swarm/other peers.
When you create a torrent file and you want to make sure it will always be seeded for a fast and sure download, you can upload that file (or folder) to an online server with public access and add its link to your torrent's metadata. If the swarm is weak (there are no or few peers), a torrent client will download the data directly from the http source. On the other hand, when the swarm is strong all the data will come from the BitTorrent network (without using your server's bandwidth) and the download itself will probably be even faster than from the server only.
How to use web seeding?
If you have your content uploaded to a server already, all you need to do is add a link to the content in the Web Seeds section in the Torrent Creation Window on FrostWire.