Two people, two very different states of being, set side by side in just four lines. The speaker looks at someone else, apparently a person they know well, who is laughing with their whole body, "from the bottom of your lungs." Then the poem turns on a single word, "And," and the speaker describes themselves: "barely breathing."
The contrast does most of the work. Both images are about breath, but one person has more than enough of it and the other has almost none. That suggests the speaker is struggling, perhaps with sadness, anxiety, or a feeling of being left behind while someone else is happy. The poem never says why, which keeps it open and a little haunting.
The mood is quiet and lonely. There is no anger toward the laughing person, just a gap the speaker can't cross. The short lines and tight pauses even mimic shallow breathing, so the form reinforces the feeling.