What is "secret cocoon"?

I read a paragraph which describes the beauty and tenderness of a woman with nobility.
It said, "She had the most vivid blue eyes, hair the colour of late summer straw, skin that shone with health and a soft, sibilant voice that seemed to enfold you in a secret cocoon, as if you and she were the only people in the world."
What could "secret cocoon" refer to….? Do native English speaker use "secret cocoon" ?