Answer:
It symbolizes learning through time and loneliness.
Step-by-step explanation:
Birches is a poem written by Robert Frost, born in San Francisco in 1874. In the poem, the speaker thinks about the past. These thoughts are originated by the birches seen by the speaker, who creates an environment with words related to nature and winter. This context could give the idea of cold and also things that can be easily broken ("Such heaps of broken glass to sweep away/You'd think the inner dome of heaven had fallen"). According to the speaker, the birches cannot be bent by a human force; however, he prefers to imagine it could happen.
The speaker refers to loneliness when he mentions that the birches are bent by a boy, described as one "too far from town to learn baseball,/ Whose only play was what he found himself/Summer or winter, and could play alone." It is also very important to remark that these actions happen while the boy takes a trip ("As he went out and in to fetch the cows"); even if it is a small one, trip in literature implicates learning, such as Odysseus or Dante Alighieri, among many others. This learning is literally mentioned when the speaker says "He learned all there was/To learn about not launching out too soon/And so not carrying the tree away/Clear to the ground. He always kept his poise/To the top branches, climbing carefully". The semantic field used in this part of the poem is related to conquer, learning, or accomplishing something.
Even the speaker has a strong wish to experience something like that again ("So was I once myself a swinger of birches./And so I dream of going back to be"), he also thinks he could come and go, matching earth and heaven using a birch as the link.