A function is called surjective or onto if every element in the target set has at least one corresponding element in the domain. This means that for every output value, there is at least one input value that maps to it. In simpler terms, a surjective function covers the entire range of possible outputs. 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. If any grade is missing, the function is not surjective, as it fails to cover the entire target set of grades.