It would be great if someone elaborated on the red stamp things

It's prison currency

tysm

I've never heard of it before.. can we buy things inside the prison with it? Or get the sentences less or something

Prison currency can't buy you a lessened sentence, that's under the jurisdiction of the higher-ups or your lawyer (but deals like a lessened sentence comes with HEAVY price and is very rare to come across if you don't have connections and I mean big-time connections). As for its use, yep. You can buy anything smuggled in the prison with it. Or you can send it back home. Or wait til your sentence is done and exchange it with real money.

Thank you for explaining in such great details (≧∀≦)