A surjective function, also known as an onto function, is a type of function where every element in the target set is mapped to by at least one element from the domain. This means that for every output value, there is at least one input value that produces it. In simpler terms, no element in the target set is left out. For example, consider a function that assigns students to their respective classes. If every class has at least one student assigned to it, then this function is surjective. In contrast, if some classes have no students, the function would not be surjective. Understanding surjective functions is important in fields like mathematics and computer science.