Refém translates to "hostage". It's a strong and dramatic word that immediately grabs attention, as it's not commonly found in everyday song lyrics.
In the song, the artist uses it metaphorically to describe being emotionally trapped by someone, even when he claims to be free. He sings, "Seu eu 'to livre, mas sou seu maior refém" (Even if I'm free, I'm your biggest hostage), highlighting the intense, almost captive, nature of his feelings despite his supposed freedom. This makes the word particularly impactful and memorable.
"Cartão" is about drowning heartbreak in spending. MC Davi is stuck on a woman who never seems to feel what she puts him through, and instead of dealing with it he goes out drinking and partying to numb it. Every night out gets billed straight to his credit card.
The chorus lays out the trade: he drinks, he spends, and none of it actually fixes anything ("O que adianta eu beber, fumar, gastar" - what's the point of drinking, smoking, spending). The bill just keeps climbing ("'Tá muito alto a fatura do coração" - the heart's invoice is running way too high), because she is the one making it grow, not the nights out.
By the end he is dropping serious money on a single night while she is nowhere around to see it. It is less about the credit card and more about how easy it is to spend your way past a feeling instead of facing it.