Robert Frost's single-stanza poem "Out, Out—" tells a fairly grim story. The poem begins with telling readers about a young boy that is old enough to be cutting stove length wood for his family. His sister calls him in for dinner, and his concentration is broken just enough for him to lose control of the saw. The saw mangles the boy's hand. A doctor is called in to amputate the hand and clean up the damage. The doctor uses ether for the purpose of anesthesia, and the boy dies.
The "I" that the question asks about can be found in line 10 of the poem. This is the moment that the poem's speaker momentarily places himself within the poem in the first person. The speaker does this in order to more deeply express his sorrow at the events that follow.
"They" is used four times within the poem, and the first time it is used is also in line 10. The speaker admits that he/she wishes "they" would have told the boy to quit his work for the day. The "they" refers to the boy's family. He is cutting wood for the family to use, and the poem's speaker knows that if "they" had told the boy to quit only a half an hour earlier, the boy would not have sustained the injury with the saw. Quitting a half an hour early would have saved the boy's life.