... which is, by and large, that they can dismiss anything said by someone who's got it better than someone else. Have more money? Whiter skin? The right genitals? You've got "privilege", and you can't see the bad stuff properly, because your life's been SO GOOD. So that thing you said is just nonsense.
This doesn't mean that privilege doesn't exist, but it is used as an argument-ending "you're just wrong" by some. In its own way, it can be equally as prejudiced and stupid as "you're just a woman so your opinion doesn't count".
People like to take the easy way out, and many feminists are no exception. Slap a label on something, pass it through the "yes/no" machine, and be done with it without having to really think about things too hard.
And then when people call you on it, just say they're privileged and you don't have to take their critique seriously. Win.