A function is called "surjective" or "onto" if every element in its target set is mapped to by at least one element from its source set. This means that for every output value, there is at least one input value that produces it. Surjective functions ensure that no part of the target set is left unused. For example, consider a function that assigns students to their grades. If every grade from A to F is assigned to at least one student, then this function is surjective. In contrast, if some grades are not assigned to any student, the function is not surjective.