Pretty sure this was made clear in the article but... I'll outline the little I know on the subject as a complete layman.

Currently we have been able to use quantum effects to create single runs of fibre that cannot be intercepted. That is, if the data is intercepted by any known means the receiver will be able to detect this.

The shortcoming of this method, is that of course when you need to amplify the signal, that's generally a "store and forward" operation and thus would also break this system's detection. You could I guess perform the same operation wherever it is amplified, but it's then another point in which monitoring could happen. If you want 1 trusted sender, 1 trusted receiver and nothing in between, this is a problem.

What this article is saying, is they have found a way to amplify the information without ever "reading" it. Therefore keeping the data integrity showing as "unseen" (for want of a better word). As such this will allow "secure" (I guess?) fibre runs of greater distances in the future.

Now the article does go into some detail about how this works and why. But, for the basic aspect of why this is a good and useful thing. This is pretty much what you need to know.