One of the first things you have to understand about the poem is what Frost is literally talking about.
Birches are stiff, but supple trees with many branches.
They are easy to climb, and if you get high enough up, you can rock them back and forth to make them sway. Do this enough times and you can sway them all the way to the ground. A very simple, but enjoyable childrens game that seems to Frost the ultimate example of childhood innocence. Here Frost dedicates the first part of the poem to a comparison between nature and humans. Obviously it is better for a boy to have bent these birches because the trees will rise again.