Overpowered (often shortened to OP) is an informal game term. Something that is overpowered is just too strong. It's a common term in games of all sorts.
Decipher often attempted to fix OP cards with errata, or put them on the X-List. Likewise, unofficial formats often X-List or errata OP cards. Fixing OP cards is one of the main goals of the PC Errata project.