The Dunning-Kruger effect describes the inverse relationship between actual competence and perceived competence at the low end of the skill spectrum. People with limited knowledge in a domain lack the metacognitive ability to recognise their own incompetence.
Important nuance: the popular internet version of this concept (the "Mount Stupid" graph) is not exactly what Dunning and Kruger found. Their 1999 paper showed that bottom-quartile performers overestimated their percentile ranking — they did not show experts becoming more uncertain than novices across all domains.
What is robustly replicated: unskilled performers lack the metacognitive access to see their own errors. This is precisely why metacognitive training matters — building self-monitoring capability is the mechanism by which you escape the effect.