There are several themes that are applicable to "Out, Out—" by Robert Frost. One is hinted at in the title which alludes to this part of Macbeth's speech:
Out, out, brief candle!
Life's but a walking shadow, a poor player,
That struts and frets his hour upon the stage,
And then is heard no more.
In this poem, Frost uses a child to echo the sentiment that life is fragile and fleeting.
The saw has a menacing characterization from the first line as it snarls and rattles. As a young boy uses it in his work, he is "all but done" with his task and the day when his sister appears to call him to supper. This sets up the first tragedy. He is startled, and the saw horrifically cuts his hand. A doctor comes, gives him ether, and the second tragedy occurs: the young boy unexpectedly dies.
Frost uses a child to make the theme more vivid. Young children aren't supposed to die. This child is doing a man's work when he is caught off guard by a simple call to supper. His sister comes to bring him news of an impending nourishment that instead ends his life. The doctor who comes to save him instead creates a medical complication that kills him.
In each section of the narrative, the unexpected happens, and all combine to create a situation that ends a young child's life. Life is full of the unexpected, and this poem indicates that life is simply a candle that spends a brief moment on stage and then "is heard no more."