How we cite our quotes: (Chapter.Paragraph)
Quote #1
Though there were few great cities on Earth where men could not see one of the silver ships glittering against the zenith, after a little while they were taken as much for granted as the Sun, Moon, or clouds. (2.103)
The power of the Overlords is symbolized in the ships and how they hover over the great cities of the world. It's the same thing as super-rich people using pools on top of skyscrapers to symbolize how rich they are. It's all about altitude, yo.
Quote #2
The invaders had brought peace and prosperity to Earth—but who knew what the cost might be? History was not reassuring; even the most peaceable of contacts between races at very different cultural levels had often resulted in the obliteration of the more backward society. (3.7)
In other words, since the Overlords have the power, they're probably the ones who are going to come out on top when all is said and done. But, hey, you never know—we'll have to see how things go.
Quote #3
"We kidnapped you, partly to show Karellen that we mean business and are well organized, but largely because you are the only man who can tell us anything of the Overlords. You're a reasonable man, Mr. Stormgren. Give us your co-operation and you can have your freedom." (3.89)
Stormgren's power lies not in his ability to command nations because, well, he can't. It's the knowledge he possesses that gives him power, and the people who kidnapped him want a taste of that knowledge-power.