it means that you are downloading it from their servers instead of the original user, your ISp doesn't care about you using bandwidth to download from their servers because their internal network has plenty of capacity and upgrades are relatively cheap (a 1 off cost in most cases to lay cables or to upgrade the equipment to use existing fiber at higher speeds or whatever)
the only real use of caches so far are for websites (for example AOL is a major ISP that caches all web traffic), when you request a page your request instead goes to your ISPs cache, that then gets the web page and sends it to you - no benefit there, however another user then connects to the ISPs cache and requests the page, it can send it from memory instead of getting it again - this reduces the bandwidth usage on their connections to other ISPs (the expensive ones to maintain and upgrade etc)
the general idea of caching for p2p is the same, the first download will be just like normal, but the next user who downloads it gets it from the ISPs server instead - this is good for the ISP because it reduces their bandwidth usage, and good for the user downloading because they get it from a faster cache, and good for other users because there is now an extra slot available on the person who was going to upload it...
web caches have some flaws (for example if a page changes and the cache has an old one) but in the case of p2p that is not an issue, all of the benefits and i can't see any problems with it - however I'm not sure how well it would work for p2p, because of the huge variety of files (anything popular has hundreds or thousands of files for the same thing... so 2 users on the same ISP picking the same one...)
if it works then great news, however it is much harder for a p2p network than for websites so I'm not sure if it will work... for torrents it could work to a certain extent, unfortunately i don't see them bothering with winmx