Comment by mmooss

They mean nuclear-powered submarines (some of those are nuclear armed, but that's mostly unrelated).

It's a very big deal because nuclear-powered submarines are very powerful weapons. Subs are about stealth, and other subs must surface regularly (daily?), greatly limiting their ability to hide. Nuclear-powered subs can stay submerged for many months and can be almost impossible to detect. They are, in a sense, unstoppable weapons.

And it's a very big deal because, as of five years ago, the US only shared this very difficult technology with one country (afaik), the UK. It is among the US's most valuable military secrets. The Biden administration shared it with Australia, apparently to cement the bond with Australia as a counter to China (Australia will require many years to develop the shipyards and capability to build their own nuclear-powered subs.)