This means that if somebody tries to copy their music to a new device it will no longer play.
Now imagine that Apple has to shut down its iTunes license servers for some reason. Maybe they are found guilty of fiddling the books and go bankrupt in 2010. Not terribly likely, but who knows what the future holds? It would basically disable millions of dollars of purchased music overnight. Most people probably don't realise that a track purchased with DRM is actually only rented.
Obviously, if you had just downloaded the track illegally, without paying the rights owners a penny then you wouldn't have any problems at all. And the music industry STILL can't understand why piracy is so popular. If you want to fight something that's free you have to do it with a better product. Trying to make people pay over the odds for something that is massively inferior to the free alternative is never going to work, no matter how many people you threaten to sue.
News on the Yahoo thing is here