Complementary cumulative distribution function as the name suggest complements cumulative distribution function (CDF). Cumulative Distributive Function (CDF) is used to find the probability of a variable taking a value less than or equal to x for any given function and one of the properties of CDF is that it goes to 1 as x tends to infinity. So, at any given point of time if we sum CDF and CCDF, we will get the resulting number to be 1.

CDF is used to find the probability of a variable taking a value less than or equal to x, CCDF is used to find the probability of a variable taking a value greater than x. In simple terms, it is used to understand how often a random variable is above a particular level. In actuarial science, the complement of the CDF is called the survival function.