A function is called bijective when it is both injective and surjective. This means that every element in the domain (input set) maps to a unique element in the codomain (output set), and every element in the codomain is covered by the function. In simpler terms, a bijective function creates a perfect one-to-one correspondence between the two sets. For example, consider the function that pairs each person in a group with their unique identification number. Here, each person has one specific number, and each number corresponds to one person, making the function bijective. This property is important in mathematics and computer science for establishing clear relationships between sets.