The Phantom Menace was a villain in Flash Gordon, the bubble screen the Trade Federation is straight out of the Buster Crabbe serials (including the sound) and the crawl in all Star Wars films is lifted from those serials.

Lucas wanted to make a series of films based on Flash Gordon in the seventies and when he couldn't get the rights he tried to write something in the same vein.

The result was ANH so presumably it was a nod to that heritage.

Contextually it refers to Sidious the allusive, something elsewhere that is manipulating everything from behind the scenes.