Mood refers to the atmosphere an author creates through the setting he/she creates, the diction he/she uses and the tone that he/she employs. The purpose is to evoke certain feelings in the reader.
In Once Upon A Time, Nadine Gordimer creates a suspenseful mood in the beginning, filled with trepidation, uncertainty and fear. The setting describes her as being alone at home during the night. She is roused by a strange sound, which makes her anxious.
The diction she uses, heightens the mood. She intersperses her sentences with short phrases which increases the tension:
And then last night I woke up—or rather was awakened without knowing what had roused me.
A voice in the echo-chamber of the subconscious?
A creaking of the kind made by the weight carried by one foot after another along a wooden floor. I listened. I felt the apertures of my ears distend with concentration. Again: the creaking. I was waiting for it; waiting to hear if it indicated that feet were moving from room to room, coming up the passage—to my door.
The tone is anxious, filled with uncertainty and fear, which adds to the suspense.
I have no burglar bars, no gun under the pillow, but I have the same fears as people who do take these precautions, and my windowpanes are thin as rime, could shatter like a wineglass.
Furthermore, the reader identifies with the author's anxiety through the diction. Gordimer uses words related to danger and a probable threat:
A woman was murdered (how do they put it) in broad daylight in a house two blocks away, last year, and the fierce dogs who guarded an old widower and his collection of antique clocks were strangled before he was knifed by a casual laborer he had dismissed without pay.
However, the reader feels the narrator's relief when the mood is lightened and the suspense is lifted, when she provides a logical reason for the sounds she hears. There is no intruder, after all.
There was no human weight pressing on the boards, the creaking was a buckling, an epicenter of stress. I was in it. The house that surrounds me while I sleep is built on undermined ground; far beneath my bed, the floor, the house's foundations, the stopes and passages of gold mines have hollowed the rock, and when some face trembles, detaches and falls, three thousand feet below, the whole house shifts slightly, bringing uneasy strain to the balance and counterbalance of brick, cement, wood and glass that hold it as a structure around me.