In A Doll's House, Torvald is married to Nora. He has several nicknames for her, including lark or skylark and squirrel. Often they are accompanied by diminutives like sweet or little.
These nicknames demonstrate how Torvald feels about Nora. He sees her as a child, as something small, delicate and not very smart. He thinks she needs to be protected.
The attitude Torvald holds about Nora is what pushes her to leave him at the end of the piece. She tells Torvald that she has been kept a child, a doll, for her whole life. She went from being a child in her father's house to being a child in her husband's. She has been protected and kept from progressing or learning. She feels that she needs to find out who she is outside of the doll's house that she has lived in her whole life.