。
She was working away at her sewing.
Literal
She [topic-は] needle [object-を] was-advancing.
針をすすめる (はりをすすめる, literally 'to advance the needle') is a set expression for making progress on sewing or needlework. The past progressive ~ていた places us in a moment when she was in the middle of working — this form often serves as narrative background, setting the scene for what happens next.