Ninar means 'to lull to sleep' or 'to cradle'. It evokes a sense of tenderness and comfort, often associated with a lullaby or a parent soothing a child.
In the song, the lyric "Me envolve em teu cheiro e assim faz ninar a imensa vontade de estar ao seu lado" uses ninar metaphorically. It suggests that the lover's scent is so comforting and enveloping that it lulls the intense desire to be by their side, implying a deep sense of peace and contentment in their presence.
Banda Eva built "Beleza Rara" as a straight love song, and every verse in it exists to say the same thing a new way. He opens by refusing to let time win: "Eu não posso deixar que o tempo\nTe leve jamais para longe de mim" (I can't let time ever take you far from me). A few lines later he gives the song its title outright, calling her "Minha pedra rara" (my rare gem) and claiming not even the sea shines the way her eyes do.
The chorus drops the metaphors and just repeats itself, which is the whole trick. "Hoje sou feliz, feliz e canto\nSó por causa de você" (Today I'm happy, happy and I sing, only because of you) comes back again and again until the words wear down to "Feliz, feliz" (Happy, happy) and nothing else. It is built to be sung back by a crowd, not read on a page.