Threshold means boundary, so it is also called critical value, which refers to the lowest or highest value that an effect can produce. Content-specific thresholds can be widely used in various aspects, including architecture, biology, flight, chemistry, telecommunications, electricity, psychology and so on, such as ecological thresholds. Mathematically, the value of the independent variable x must be within the definition domain of the function, and the dependent variable y must be equal to a certain value, and the definition domain of this function is the threshold value of X.