The role of an SSBN and a context for being forward deployed can be found by reading about the Nuclear Triad: https://en.wikipedia.org/wiki/Nuclear_triad

In the US context, SSBNs are one prong of this triad, and the platforms are positioned to always be at the ready to carry out strike orders when called upon by the Nuclear Command Authority. These hulls are equipped with nuclear reactor power plants, so fuel for the sailors (i.e. humans) becomes the biggest constraint for how long a submarine can remain underway.

National Command Authority?

Yep --typo-- Thanks for pointing it out!

NCA is the National Command Authority (President as Commander-in-Chief and the SECDEF) https://en.wikipedia.org/wiki/National_Command_Authority

Boomers on the mind :)

Meaning on station, performing a task, not just on patrol and free to take a break to come into port.