Desarrumar means 'to mess up' or 'to disarrange'. It's an interesting word because it's the opposite of 'arrumar' (to tidy up) and implies a state of disorder or chaos.
In the song, Maria Rita uses it in the line "Não deixe que o mau tempo venha pra desarrumar" (Don't let the bad weather come to mess things up). Here, 'mau tempo' (bad weather) is a metaphor for difficulties or problems that could disrupt the harmony and order of a relationship. It's a vivid way to express the desire to protect something precious from falling apart.
Maria Rita sings about a couple patching things up after a rough patch. She opens with "Tenho tanto pra falar" ("I have so much to say") and asks her partner not to let bad weather wreck what they've rebuilt: "Não deixe que o mau tempo venha pra desarrumar" ("Don't let the bad weather come and mess it up"). The reassurance keeps building through the verse — "Isso é coisa de momento, eu sei que vai passar" ("This is just a passing thing, I know it will pass") — until the chorus commits to forever: "Rumo ao infinito vai" ("It's heading toward infinity"), because their love "balança mas não cai" ("sways but doesn't fall"). The hook "Tá na hora da gente com jeito reconciliar" ("It's time for us to gently make up") is the whole song's request in one line — not a dramatic reunion, just a quiet, patient one.