If something's tainted, it's ruined or spoiled. If you leave milk on the counter overnight, it could be tainted. But a charity that uses its funds to buy board members tropical vacations could also be considered tainted.
The adjective tainted describes a person or thing that's been touched by rot or corruption. Many think that young minds can become morally tainted by violence on TV. If evidence at a crime scene is handled improperly, it could be considered tainted and can't be used at trial. You've probably heard about foods like spinach and peanut butter being recalled because they have been potentially tainted with salmonella.