Final answer:
In 'Mending Wall,' 'them' refers to the fallen boulders that the speaker and his neighbor repair every spring.
Step-by-step explanation:
In the poem Mending Wall by Robert Frost, the word 'them' in line 6 refers to the boulders that have fallen from the wall that divides the properties of the speaker and his neighbor. Every spring, during the mending-time, they find the boulders that have tumbled down due to natural processes or the hunters' actions.
The act of picking up the boulders and repairing the wall serves as a metaphor for the broader themes of boundary maintenance and relationship dynamics explored in the poem.