From the wikipedia page: In the field of psychology, the Dunning–Kruger effect is a cognitive bias in which people with low ability at a task overestimate their ability. It is related to the cognitive bias of illusory superiority and comes from the inability of people to recognize their lack of ability. Without the self-awareness of metacognition, people cannot objectively evaluate their competence or incompetence.

> People who are actually bad at what they do rarely realize it

Apparently, they may often think that they are good at it! See https://en.wikipedia.org/wiki/Dunning–Kruger_effect