The Dunning-Kruger effect occurs when individuals don't properly analyze their personal levels of competence. This is a cognitive bias in which incompetent people tend to think they are more competent than they actually are in a situation. It is theorized that this occurs because without much knowledge about a situation or a subject it is difficult to adequately self-analyze competency therefore leading the individual to think they are more competent than they actually are.
The opposite of this is imposter syndrome which is when very competent individuals tend to think they are less competent than others. Common examples of the Dunning-Kruger effect are politicians and celebrities who make bold incorrect claims about scientific matters without fully understanding the subject, like climate change or vaccinations. As Charles Darwin once said "Ignorance more frequently begets confidence than does knowledge."