Typically it involves some entity which can use traffic-data to strip away the privacy people think they have.
A simple example might be to determine the true identity of an HN poster: Even with the content encrypted, it's harder to hide the fact whenever Account X makes a post, Person Y is always sending packets to HN.
A simple example might be to determine the true identity of an HN poster: Even with the content encrypted, it's harder to hide the fact whenever Account X makes a post, Person Y is always sending packets to HN.