Pamela Hobart Carter starts with a playful question: what if each raindrop needs the exact patch of sidewalk where it falls? From there the poem builds a chain of dependence. Skies need clouds, the speaker needs an umbrella, and the umbrella needs raindrops to give it a purpose and a shape. Nothing here stands alone.
Then the speaker turns to the poem itself. By saying "We are all here because I put us here," the speaker admits to being the one who made this little world, and needs it as a place "to put things in." The ending turns toward the reader with a gentle question: does the poem need you, or do you need it? The umbrella, oddly, "knows" the answer.
The mood is light, curious and warm. It feels like a quiet thought spoken aloud, and it suggests that poems, like umbrellas, shelter us a little.