In this poem, the theme of loss is tied closely to the idea of the brevity or fragility of life. The boy dies of a freak accident when his hand is cut by a saw. This accident doesn't necessarily have to be life-threatening, and at first, the boy is only concerned with the possible loss of his hand because it means that he will not be able to continue working. But the ether the doctor uses for anesthesia ultimately stops his heart.

It is interesting that the family does not grieve for the boy in the poem. It says, "And they, since they / Were not the one dead, turned to their affairs." This coping with loss by moving on reads as callous and heartless. However, the poem was published in 1916, during WWI. Death of young men was a huge part of life, and one way that people deal with that kind of horror is to continue working and going about their lives rather than stopping to process the pain and loss.

In "Out, Out—," Robert Frost explores the theme of loss in a number of ways. Firstly, he uses the title of the poem to suggest that life is very fragile and that it is easily lost. To do this, Frost employs an allusion from Shakespeare's Macbeth in which Macbeth uses the phrase "Out, out, brief candle" as he is musing about the death of his wife, Lady Macbeth. By naming his poem after this phrase, Frost shares Macbeth's belief that life is little more than an illusion and that it can be put out (just like a candle) at any moment.

In addition, Frost also explores the theme of loss from the boy's perspective. In this case, it is a loss of innocence which Frost touches upon: the young boy is out cutting wood for his family because he has to contribute to their survival. This is a significant burden for the young boy and he wishes to be freed from it, as shown by the following line:

"That a boy counts so much when saved from work."

Similarly, it is the boy's young sister who calls him in for supper, which hints at her domestic contribution to family life. Like her brother, the young girl is forced to help her family and, therefore, she has also lost her innocence.

Finally, Frost also explores loss in a literal sense: the boy loses his hand in an accident with a saw and, as a result, he also loses his life. To emphasize this sense of loss, Frost describes the boy's deathbed scene, in which he is surrounded by his family as he draws his last breath. Frost deals with the boy's death in a dark, yet pragmatic, way: the boy's family and the villagers do not spend time mourning his death and instead return to their daily lives, too consumed with the business of survival, as we see in the closing lines:

"Little—less—nothing!—and that ended it. 
No more to build on there. And they, since they
Were not the one dead, turned to their affairs."