Used to represent the result of multiple derivatives of a function. The apostrophe represents the first derivative, that is, the slope of the function; Twice represents the second derivative, that is, the curvature of the function, and so on, three times represents the third derivative, four times represents the fourth derivative, and so on. Higher derivative can provide more information about the change rate and curvature of function curve, which is very useful for studying the properties and behavior of function.