Bilhetinho is the diminutive form of bilhete, meaning 'note' or 'ticket'. The suffix -inho adds a sense of smallness, endearment, or casualness, making it a 'little note' or 'sweet note'.
In the song, the narrator recalls a time when their loved one wrote them "Em um bilhetinho" (In a little note), hinting at a personal, intimate message that holds sentimental value. This word evokes a feeling of nostalgia and tenderness, making it quite engaging.