Karen Bassey uses a butterfly to talk about wanting something precious that can't be possessed. The opening lines describe it as beautiful but rare, something you don't get to see or hold very often. Even when you love it and badly want it, the speaker says, you can't count on having it, and when you finally do, you can't trap it in your hands.
The last line is where the poem shifts. Until then it seems to be about a butterfly, but "him" suddenly reveals that the butterfly stands for a person. The judgment is that you can't keep him locked up in your heart, which suggests the speaker is talking about love, and perhaps about letting someone go. The poem doesn't say whether this comes from loss or from acceptance.
The mood is quiet, gentle and a little sad. The short, plain lines sound like someone reminding themselves of a hard truth: loving something deeply doesn't give you the right to hold on to it.