What is meant by leaving at the beginning of the novel?

Kira is referring to her mother's spirit leaving her body.

There was no reply. She hadn't expected one. Her mother had been dead now for four days, and Kira could tell that the last of the spirit was drifting away.
"Mother." She said it again, quietly, to whatever was leaving. She thought that she could feel its leave-taking, the way one could feel a small whisper of breeze at night.