It means that for any element x 1 in the set d 1, there is an element x2 belonging to the set d2, that is, each element in d 1 can be mapped to an element in d2 in some way. This expression is very common in the concepts of function and mapping in mathematics. Suppose d 1 is a real number set and d2 is a square number set of a real number set. For any real number x 1 belonging to d 1, we can find a corresponding square number x2 belonging to d2, so that x2 is equal to the square of x 1. Every element of d 1 can be mapped to an element in d2 by squaring, satisfying the given conditions.