The speaker is clinging to something, or someone, that other people refuse to believe in. The "you" is never explained, which makes it feel half-present, "not real but there." The speaker admits that the doubt of others crept inward and made them question their own mind, but the poem ends up on the side of stubborn certainty: "I know your real." That line is repeated, as if the speaker is reassuring themselves as much as anyone else.
The mood is tender but fragile. There is loneliness in being disbelieved, and a protective instinct in the fear that others would "corrupt" what is precious. The title, with its image of throwing flowers out the window, hints at something discarded or an ordinary gesture left behind, though the poem itself doesn't spell out the link. The final lines turn "dream" in two directions: the "you" is dreamlike, and also the thing the speaker most wants. The plain, unpolished wording makes the feeling seem honest and raw.