The Dunning-Kruger effect is the phenomenon that people who are incompetent in a certain area often are not aware -and cannot be aware- of just how incompetent they are. A explanation of this counter intuitive effect is the following: when you know little about a subject you do not know how much knowledge you are lacking due to which you are likely to overestimate yourself.
The Dunning-Kruger effect can cause other people to be surprised and irritated when they see someone who is not so competent but who behaves as if he or she is. It can also lead to surprises and confusion in the person him or herself. This is especially the case when someone tries to master new knowledge.
I came across the following picture which illustrates some of the experiences one can encounter when learning something new. The picture shows that learning something new can feel like sliding down a ramp.
By the way, not everybody is equally vulnerable for the Dunning-Kruger effect. Particularly people in a fixed mindset have a tendency to overestimate themselves in areas in which they are not knowledgeable (read more).