From the Wikipedia:
"The Dunning–Kruger effect is a cognitive bias in which relatively unskilled individuals suffer from illusory superiority, mistakenly assessing their ability to be much higher than it really is. The bias was first experimentally observed by David Dunning and Justin Kruger of Cornell University in 1999. Dunning and Kruger attributed this bias to a metacognitive inability of the unskilled to recognize their own ineptitude and evaluate their own ability accurately. Their research also suggests corollaries: highly skilled individuals may underestimate their relative competence and may erroneously assume that tasks which are easy for them are also easy for others.
Dunning and Kruger have postulated that the effect is the result of internal illusion in the unskilled, and external misperception in the skilled: "The miscalibration of the incompetent stems from an error about the self, whereas the miscalibration of the highly competent stems from an error about others." (https://en.m.wikipedia.org/wiki/Dunning%E2%80%93Kruger_effect)
See: Illusory Superiority