Frost's title is a reference to Shakespeare's Macbeth. It alludes to Macbeth's speech after the death of his wife, where he comments on the frailty and pointlessness of life. The full text is:
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. It is a tale
Told by an idiot, full of sound and fury,
Signifying nothing.
Depressing, huh? We know. We're here to help. Why would Frost use this title? Well, for one thing, the poem is exploring similar themes of human helplessness. We're dealing with life cut off, abbreviated, and taken away here; slicing the whole phrase up (so "Out, Out" as opposed to "Out, out, brief candle!") extends that image.
In Macbeth, Lady Macbeth is also torn up by her part in the murder of Duncan. She has blood on her hands, and can't get it off (she echoes the above passage when she tries to clean herself in scene 1 of Act 5, saying "Out, damned spot! out, I say!"). Ultimately, the only way to get the blood off for her is through suicide—the imagery of the boy's hand matches that of Lady Macbeth's. The boy is too young to be careful in his work, and so he dies. The work of their hands is too much for them to handle, and death is a way out of sorts.
The title, then, brings together these ideas of work, death, and the meaninglessness of life itself. Good times?