You have asked an excellent question that relates to the ending of the story, the symbolism of the light that Granny Weatherall blows out and also the importance of her jilting to her life and how she views God and death. Before answering your question, let us remember how Porter ends this excellent story:
For the second time there was no sign. Again no bridegroom and the priest in the house. She could not remember any other sorrow because this grief wiped them all away. Oh, no, there's nothing more cruel than this - I'll never forgive it. She stretched herself with a deep breath and blew out the light.
Porter ends this story in this particular fashion because she wants to suggest that we are all "jilted" in death - that we die alone and that this solitude is greater than any loss we know in life. It is also important to remain aware of what she is suggesting about Granny Weatherall's character through her final acceptance of this reality - Granny Weatherall is clearly shown as a strong characters in accepting her own death in this face of this ultimate jilting. The light that you identified symbolises the last flicker of her consciousness. Granny Weatherall's action in blowing out this light indicates Granny giving herself up to death and her relinquishing of this consciousness and life.