The tough part about "Birches" is following the narrative. For the first read-through you might not have any idea what it means to swing a tree. Also, if you've never spent time in a cold and wooded environment, you might have to do some imaginative work to visualize the details of the scenery and subject matter. If you can't figure out if the speaker is talking about death, imaginary heaven, or just climbing trees, don't worry. It's supposed to be vague.