I'm afraid I don't completely understand your question -- why the things in parentheses? I'll try to answer, let me know if you want to clarify what you mean.
The setting of the story in Greenwich Village in New York City. This was a place where many artists and people like that lived in these days.
Johnsy's fancy (fancy meaning something one imagines, in this case) is that she will die when the last leaf on the ivy vine falls.
This is the significance of the title. It refers to the idea that she would die when the leaf fell and it refers to the leaf that Behrman painted. This is Behrman's masterpiece -- the leaf that was so life-like that it convinced Johnsy to live.
In the story "The Last Leaf" Johnsy and Sue are both artists that are living as roommates in Greenwich Village, New York. Johnsy becomes sick with pneumonia because of the winter months. She lies in bed and begins to will herself to die as her heath worsens. Johnsy has been watching a vine of leaves steadily losing their leaves everyday. She believes that when the last leaf falls off of the ivy vine she will die.
If I were to say what was Johnsy's fancy I would state that it is watching the leaves falling from the vine as she waits to die. She refuses to eat or do anything to help improve her health because she is fixated on dying when the last leaf falls.
Old Behrman, a painter who resides downstairs and is past 60 years old, finds out about Johnsy's thoughts. Late one night he sneaks out and paints a leaf to match the last one on the wall so it looks like it is still attached. Success has always slipped away from him. He has always wanted to paint a masterpiece.
Johnsy watches the leaf cling to the vine. She is unaware that it is painted. She gets better because she sees that the leaf is still there. She recovers but Mr. Behrman dies. He ahs painted his masterpiece that gave Johnsy life.