The poem reads like a small piece of advice, almost a blessing, built on the life of a butterfly. It starts with a caterpillar, who is told to hope for change, to become something lighter and more free. Once that happens, the hope shifts. A butterfly can't ask for much more time, so the wish becomes about the quality of what remains: "fourteen days" lived as "a good journey."
The speaker sounds gentle and tender, like someone talking to a friend who is facing something hard. The title, "Fallen angel," and the note "last fourteen days" suggest that the butterfly may stand in for a person, though the poem never says so directly. Judging by the imagery, the speaker accepts that time is short and doesn't argue with it. Instead they hope for a sweet, peaceful ending.
The mood is quiet and soft, with a trace of sadness, but it's calm rather than bitter. The last image, flying "into the sky," turns the ending into a release rather than a loss.