ということだ

ということだ has two jobs: it reports something you heard ("I hear that", "apparently"), and it states what a fact amounts to ("that means"). Both follow a plain-form sentence. A noun or な-adjective takes だ before it, never な: ×静かなということだ. In the "that means" use a noun often drops the だ.

Hearsay

ということだ reports information from another source without the speaker vouching for it. The source often comes first with によると or では. It is more formal than そうだ; you hear it in the news and in announcements, and in conversation んだって does the same job.

"That means"

Here ということだ gives the conclusion you draw from a fact. The fact often comes first as ということは, and つまり or ようするに要するに can introduce the conclusion. As a question – ということですか – it checks that you have understood.

In casual speech the "that means" use shortens to ってこと: それって、いかない行かないってこときごう？