In his poem "Birches," Robert Frost makes the point that life in this world is not easy sometimes. It is often "too much like a pathless wood." One's face gets all covered in cobwebs, and one gets hit in the face by twigs, making one's eyes weep. The speaker would very much like to escape these tribulations, at least for a little while, and then return to real life refreshed and ready to start again.
The speaker would like to go back to the days of his youth when he used to climb birch trees, getting closer and closer to Heaven, until the tree could not bear his weight and would dip down in a rush and set him on the ground. A boy who does this is free from care and focused on his play. He can experience the sensation of flying, the exhilaration of controlled risk. He also learns how to avoid launching too soon, which would break the tree, but rather to climb as carefully as he can to the top before he swings down to the ground "with a swish." This simple enjoyment provides a momentary escape from the pressures and struggles of life.
Yet there is a deeper meaning in swinging from a birch tree. The action suggests overcoming one's limitations. The swinger climbs to the very top of the tree, pushing the limits of how high a person can go and feeling the risk involved. When he can climb no further, he rides the tree down, getting as close to flight as possible and challenging human limitations. This action of birch swinging is "good both going and coming back," the speaker asserts. It is good to reach up for better, higher things. It is good to experience the rush of joy and excitement that life can offer. Indeed, "One could do worse than be a swinger of birches."