Tudim is a colloquial, diminutive form of tudo, which means "everything." It's a charming and informal way to say "everything little thing" or "absolutely everything."
In the song, the lyric "Eu te roubo um beijo e devolvo tudim" (I steal a kiss from you and give back absolutely everything) implies a playful exchange where the singer gives back more than just the kiss, perhaps all his affection or his entire being. It adds a touch of intimacy and endearment that a simple tudo wouldn't convey.
This song is all about that amazing feeling when you're so smitten with someone, you just want to be a part of every single aspect of their life! Gusttavo Lima, with Rick & Rangel, sings about how this person makes him feel completely free, even though he's totally