Rapaz literally translates to "boy" or "guy". While it might seem like a common word, its usage here is particularly interesting and engaging.
In this song, it's used as a friendly, almost intimate address, like saying "hey man" or "my friend" at the very beginning of the song: "Eu que agradeço, rapaz" (I'm the one who's grateful, man). This informal and warm opening sets a personal tone, making it a memorable and authentic piece of Brazilian Portuguese slang that adds a unique flavor to the lyrics.
"Sonho Lindo" is João Gomes' thank-you to a love that arrived like a dream. He opens by thanking the person he's singing to for a meeting that felt this good, and admits he'd been needing exactly this kind of connection.
From there the song stays in that grateful, physical kind of love: wanting to touch, to kiss, to hear "yes." The hook keeps returning to the same idea in different words — this person is the beautiful dream he dreamed up for himself, and he wants the feeling to last until the end.
It's a simple, repetitive piseiro love song built for a crowd to sing along to, which is exactly what happens on this live recording: the audience takes over the chorus with him.