They talk about a "cleavage site", meaning a 3D shape with the chemical properties necessary to cut something. And apparently, that ability to cut molecules similar to furin enables this virus to more efficiently enter cells, i.e. successfully infecting someone with a lower dose of live viruses.
Where this new shape came from, they do not know. But since viruses tend to mutate a lot and integrate RNA snippets floating around in their host's blood stream, it is likely that they picked it up from some other virus simultaneously infecting the same individual.
In short, the new nCoV seems to be so contagious because it merged in a new feature from a different virus species.
The implication is that by specifically blocking this thing that nCov has but older Cov do not have, one might be able to drastically reduce its contagiousness.