Stripped down to a few words per line, this poem reads almost like a whispered thought. The speaker says that his mother is the "only one in the whole world," and the line breaks make us slow down over each small phrase, as if the feeling is being discovered as it's spoken. The sentence itself is simple and tender: no one else is like her.
Then comes the surprise: "byebye / biology." The childlike "byebye" sounds playful, but it also seems to wave away the scientific, factual way of describing a mother, as a biological parent. The speaker appears to be saying that what she means to him is bigger than that label. The poem can also be read as a quiet farewell, since "byebye" suggests someone leaving, though the text never says so outright.
The mood is intimate, affectionate, and a little wistful. The short lines and plain words give it the sound of a child's voice, which makes the love feel direct and unguarded.