Well, now you are getting in to ancillary material territory as to "how it works." Yes, the Fleet is mentioned in Hux's speech, and is indicated to be the primary reason why the First Order would not attack the New Republic head on.How does that even work though? We only see the one Starkiller attack which seems to be focused on a handful of planets located pretty close to each other. The beam also took time to propagate through space.
Was the fleet just sitting there not doing anything? Why was a portion of the fleet large enough to cripple the Republic if it was lost all in one place? This is like the opposite of the Star Trek "only ship in position to intercept" problem.
And why did TFA not show the fleet itself being attacked, or even mention it, IIRC?
The fleet had no reason to be doing anything at the time. The First Order was not regarded as a threat, and they were not expected to have the large amount of military assets they end up having. Poe's expression on being aboard "The Finalizer" at the beginning of TFA is one of shock at the size of their ship.
But, I agree it could have been explained a little bit better. But, there are enough elements there to keep me engaged. Kind of a reverse prequel trilogy for me; the books of the PT actually made the PT almost worth watching. The ST made me want to read more.
Mileage will vary.