self is a reference to the current instance of a class and is used to access variables and methods within the class. It must be explicitly defined as the first parameter in instance methods but is automatically passed by Python when the method is called. Using self, each object of a class can have its own unique attributes and behaviors, allowing for object-oriented programming principles like encapsulation and inheritance. For example, in a class Person, defining self.name = name inside the __init__ method ensures that each instance of Person has its own name attribute. Without self, variables would be treated as local to the method instead of belonging to the instance. Thus, self plays a crucial role in maintaining the integrity and individuality of objects within a class.