The poem is a quiet reassurance about love. It promises that someone will eventually arrive who gives you an entire galaxy when all you ever needed was one planet. The speaker seems to be talking to a person who has been hurt or has settled for less, perhaps someone who has met people who offered too much, too loudly, or in the wrong shape.
The image does a lot of work. A galaxy is huge, dazzling and impossible to hold. A planet is small and specific, a place you can actually live. The poem suggests that grand gestures aren't the same as what a person truly needs, and that being overwhelmed isn't the same as being loved. The "someone" isn't wrong or cruel, just mismatched.
The mood is gentle and a little wistful, with a hint of caution. The one-word lines slow the reading down, so each word lands separately, almost like a hesitant breath. That pacing makes the final line feel simple and settled, as if the speaker is saying something they've learned the hard way.