IQ tests scores are categorized according to how well a person does compared to the average person. Thus, a score between 40 and 54 indicates severe mental challenges; a score between 85 and 114 indicates average intelligence; and a score between 160 and 175 indicates extraordinary genius.
A bell curve shows what faction of the population fits into each category. Thus, those with a score between 85 and 114 constitute the largest category, and those with scores between 40 and 54 and between 160 and 175 constitute the smallest categories.
While IQ tests are very widely conducted and cited, some argue that IQ tests are not grounded in any well-established theory of how the brain operates, and are actually at odds with contemporary theories on what intelligence actually is.