A constructor is a special type of function in programming that is used to create and initialize objects. When a new object is created from a class, the constructor is automatically called to set up the initial state of that object. This can include assigning values to properties or performing any setup tasks needed for the object to function properly. In many programming languages, such as Java, C++, and Python, constructors can take parameters, allowing for more flexible object creation. They help ensure that objects are created with valid data and can simplify the process of instantiating complex objects by encapsulating the initialization logic within the class itself.