Pupila means "pupil", the part of your eye that changes size. It's a beautiful and uncommon word for a love song, and it's also the title of this track.
In the lyrics, the singers repeat the phrase "faz minha pupila dilatar" (makes my pupil dilate). This refers to the real biological reaction where our pupils widen when we see someone we are attracted to. The song cleverly turns this subtle physical sign into a powerful and poetic confession of a crush.
This song is the sweetest back-and-forth between two people who are completely crushing on each other but are too shy to say anything. It's a duet where they both sing about their secret feelings, and the fun part is that they're thinking the exact same things! He wonders, "Como que eu vou dizer pra ela / Que eu gosto do seu cheiro / Da cor do seu cabelo?" (How am I going to tell her / That I like her scent / The color of her hair?). At the same time, she's thinking, "Eu quero dizer pra ele / Que a rima fez efeito / Agora eu penso o dia inteiro" (I want to tell him / That the rhyme worked / Now I think all day long).
They both feel this intense, physical attraction, summed up in the brilliant line they share: "Que ela faz minha pupila dilatar" (That she makes my pupils dilate). It's such a perfect, scientific way to describe being smitten. The song details all the little schemes and daydreams that come with a new crush, like when she admits, "Ensaio no espelho pra tentar ligar / Invento mil acasos pra te esbarrar por aí" (I rehearse in the mirror to try and call / I invent a thousand accidents to bump into you out there). It's a super relatable and charming song about that exciting, nerve-wracking moment right before you find out if the other person feels the same way.