Qualified immunity more or less means that the cops can't be held directly liable for something that the courts haven't yet found to be wrong for a police officer to do while in the course of their duties. So, if a cop does something obviously wrong and fucked up in the course of their duties (like, say, detaining you in a car parked on railroad tracks) and you suffer injuries from it, but a court hasn't previously found that exact situation to be a wrong thing for a police officer to do, qualified immunity prevents them from being held personally accountable. The next person who gets detained on railroad tracks is covered, but you're shit outta luck.

I know what QI is about, the comment has more to do with fighting the cops in court when courts meet all manner of egregious police behavior with little more than stern finger wags and exasperated sighs at best (often. Very rarely, they actually do get held accountable) and endorsement at worst.