IQ, the abbreviation of Intelligence Quotient, is a measurement of a person's reasoning ability. In short, it is supposed to assess the extent to which a person can use information and logic to answer questions or make predictions. IQ tests begin to assess this by measuring short- and long-term memory. They also measure a person's capacity to solve puzzles and remember information they have heard - and the speed at which they do it. Calibration determines the mean (or expected value) and standard deviation, and the prior probability distribution associated with these constraints in the Bayesian inference (i.e. the only method that does not introduce "additional information") is the Gauss curve. Therefore, it is on this that the test is calibrated. All tests set the mean at 100. The standard deviation is usually set at 15 (in this case it is a standard IQ).
A person's IQ can be calculated by having them take an intelligence test. The average IQ is 100. If you obtain a score over 100, you are more intelligent than the average person, and a lower score means you are (a little) less intelligent. The standard deviation between 85 and 115 indicates that, as it is normally distributed, 68% of the population is situated within a standard deviation from the mean, and 95% of the population is situated between 2 standard deviations (see figure).