In the field of psychology, the Dunning–Kruger effect is a cognitive bias in which people assess their ability as greater than it is. It is related to the cognitive bias and comes from the inability of people to recognize their own lack of ability.
Often low ability people do not have the skills needed to recognize their own incompetence. Poor self awareness leads them to overestimate their capabilities (Mt. Stupid). But as experience with a subject increases, confidence typically declines to more realistic levels. As people learn more about the topic, they begin to recognize their own lack of knowledge and ability (Valley of Despair). Then as people gain more information (Slope of Enlightenment) and actually become experts on a topic (Plateau of Sustainability), their confidence levels begin to improve once again.
So what can you do to gain a more realistic assessment of your own abilities and to help others that are making this mistake?
Keep learning and practicing. Instead of assuming you know all there is to know about a subject, keep digging deeper. This helps you see that you have more to learn. If you have a boss or co-worker who is in "Mt. Stupid" and you need them to get past it to achieve your goal, then you can help them move along the curve by doing more research and helpfully and respectfully sharing more information on the subject. The more they know the less they will assume they are an expert.
Ask other people how you're doing. Ask those you trust and who you can be vulnerable with for constructive criticism. It will give you good insights into how other see your abilities rather than you assuming them. In the same way, you as a boss or a co-working can give constructive feedback (see another tool Radical Candor) to help others get to a place of enlightenment on a subject.
Question what you know. It can be easy to only pay attention to things that confirm what you think you already know, called confirmation bias. To minimize this, keep challenging your beliefs and seek out information that challenges your ideas.