The Dunning-Kruger Effect is the relatively simple observation that people lacking in knowledge on a certain subject will overestimate their own abilities related to that subject. Or rather, stupid people aren't smart enough to know they're dumb. Basically, the less you know, the more you think you know. This can have real problems when it comes to things like public health and representative democracy.
They say it effects everybody, so who knows how much we just glossed over writing this description.