The speaker is talking to someone they love, and the request is simple: keep existing. The title suggests a prayer said at night, a kind of bedtime plea, and the poem has that hushed, pleading quality. "For heaven's sake" adds a note of urgency, as if the speaker is half-joking and half-desperate.
What makes the poem work is its play on the word "matter." In the physical sense, matter is anything that takes up space. In the emotional sense, to matter is to be important. The speaker joins the two ideas: you take up space because you are matter, and you matter to me more than anyone. Even the fact that this person is physically present, simply occupying a spot in the world, becomes something precious.
The mood is tender and quietly anxious. Beneath the gentle wordplay there seems to be a fear of loss, and the closing "my love" arrives like a soft exhale after the plea.