Question

what do you mean by the expression "an expanse of pure serenity

Answer 1

It means the sparrows' collective chirruping made a noisy ambience around the grandmother. This expression 'Frivolous Rebuke' in the chapter, The Portrait of Lady means the grandmother would scold the sparrows lovingly when they sat on her shoulders or at times head.

Answer 2

An expanse of pure serenity This simile is used to describe the calmness and peaceful character of the author’s grandmother, as compared to the winter landscape in the mountains. It draws a picture of contentment.