In this poem, the speaker is relaxing in his sister's garden. For half a hour, he has been watching a butterfly sitting "poised" on a yellow flower. The butterfly hasn't moved, a fact the speaker emphasizes by stating twice that it has been "motionless!"
Speaking to the butterfly, the speaker says that once the breeze starts, the creature will fly away, called to the "joy" of flying. While it is still motionless, however, the speaker continues to address it. He says it is always welcome in his sister's garden, where it can again come and rest, as if in a sanctuary. Personifying the butterfly, the speaker invites it to listen to his conversations with his sister, as if it is a fellow human being.
The speaker realizes his time with the butterfly is fleeting and fragile. He continues to address it, a literary device known as apostrophe. He realizes this extended moment of stillness cannot last much longer. He speaks to it again, his emotions reaching a high point as indicated by the exclamation points, when he asks the butterfly to stay a little longer. He calls it the "historian" of his childhood, because it brings back old memories of chasing other butterflies with his sister. He longs to prolong this beautiful moment with it.