| Quote #1
But the force that lies behind colour and movement would escape [Adela] even more effectually than it did now. She would see India always as a frieze, never as a spirit, and she assumed that it was a spirit of which Mrs. Moore had had a glimpse. (1.5.48)
Adela is convinced that the "real" India will always elude her because she will always see India through the screen of official English culture. She's not far wrong as she constantly butts heads with Ronny about getting to see more of India.
| Quote #2
But nothing in India is identifiable, the mere asking of a question causes it to disappear or to merge into something else. (1.8.43)
The narrator suggests here that India can't be "identifiable" – i.e., packaged up in a neat definition. It will always elude description. As in other passages (see Quote #3), the word "nothing" is constantly associated with India, as if nothing could possibly express the true identity of India – if there is such a thing.
| Quote #3
[The visitor] finds it difficult to discuss the caves, or to keep them apart in his mind, for the pattern never varies, and no carving, not even a bees'-nest or a bat distinguishes one from another. Nothing, nothing attaches to them, and their reputation – for they have one – does not depend upon human speech. It is as if the surrounding plain or the passing birds have taken upon themselves to exclaim "extraordinary," and the world has taken root in the air, and been inhaled by mankind. (2.13.4) [see later: "Nothing is inside them …nothing, nothing would be added to the sum of good and evil]
The Marabar Caves have often been taken to be a metaphor for India in general. This is a figure of speech we call a "synecdoche," when a part of something (the caves) stand for a whole (India). As in Quote #2, the caves are indescribable, and there's that word "nothing" again. But just because they're nothing doesn't mean that they aren't anything. That is, it's because we don't have a conceptual handle on the caves that they happen to be – "extraordinary."