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 part of the target set is left out. For example, consider a function that assigns students to classes. If every class has at least one student enrolled, then the function is surjective. In contrast, if some classes have no students, the function is not surjective. Surjective functions are important in various fields, including mathematics and computer science.