The Dunning-Kruger effect occurs where people fail to adequately assess their level of competence — or specifically, their incompetence — at a task and thus consider themselves much more competent than everyone else. This lack of awareness is attributed to their lower level of competence robbing them of the ability to critically analyze their performance, leading to a significant overestimation of themselves.
Put more simply, it explains the phenomenon where someone is too ignorant about something to be able to realize how ignorant they are about it.
The effect is named after David Dunning and Justin Kruger of Cornell University whose research showed that people with the least skill are actually the most likely to overestimate their ability.
In their paper they mention the criminal case of McArthur Wheeler, who rubbed his face with lemon juice before proceeding to rob banks. Unfortunately for him, he believed that the lemon juice made his face invisible to the security camera - He was sincerely surprised when he saw the surveillance tapes!
In Dunning’s words:
when people are incompetent … they suffer a dual burden: Not only do they reach erroneous conclusions and make unfortunate choices, but their incompetence robs them of the ability to realize it.
This chart below depicts the effect (beware it’s not actually to scale), and reinforces the idea that having only a little bit of knowledge can be a dangerous thing: