What does don't torrent over Tor mean?

"Don't torrent over Tor" is for two main reasons: 1) it uses lots of the Tor network's resources 2) it isn't effective in providing anonymity for the torrenting because BitTorrent applications do not make effective use of the protection Tor provides. The download page puts it thusly:

Can You Send BitTorrent over Tor?

Sending your BitTorrent traffic through the Tor network would overload it even more. It isn't designed to handle such things -- the Tor network has much less capacity than it has users wanting to use it. And since it's zero-sum, every person trying to BitTorrent over Tor means many more people in Syria who can't get to their Facebook pages.

How do I download a BitTorrent file over Tor?

Feel free to visit a bittorrent website using Tor Browser, and download .torrent files from there. It's perfectly safe and not frowned upon. Then you open the .torrent file in your bittorrent program and download it. It will not download over Tor.