Here’s the Dunning-Kruger effect in a nutshell:
Incompetent individuals lack the metacognitive skills that enable them to tell how poorly they are performing, and as a result, they come to hold inflated views of their performance and ability.
Justin Kruger and David Dunning
Oddly the reverse may also be true, and highly competent individuals may have a bias towards underestimating their own ability.
If you struggle with accurate self-assessment—or if you struggle with other people’s ability to self-assess—then this is an important study to read: Unskilled and Unaware of It: How Difficulties in Recognizing One’s Own Incompetence Lead to Inflated Self-Assessments