It's, in layman's terms, server space you rent out and can use for your downloads. torrent clients like transmission are usually built in. I pay £5 a month for 2tb space and enough bandwidth to keep a good ratio.

So instead of downloading to my pc, I download torrents to the seedbox (at a blazing fast speed) and then download files from there to my pc whenever I feel like it, again at stupid good speeds.

I have FTP access, so I've just added it as a folder/mounted drive on my windows and Linux mint file explorers. Works great. Also have it on my smart TV as a web address for quick streaming of whatever I downloaded.

Edit: this also eliminates the risk of downloading Torrents locally.