In Python, a metaclass is a class that is used to create a class. When a class is defined, the class definition is passed to the metaclass, which processes it and returns a new class object. This new class object is then used to create instances of the original class. Metaclasses are used to customize the behavior of a class. They can be used to add methods or attributes to a class, or to change the way a class is created. Here is an example of how a metaclass could be used to add a method to a class:

class MyMetaclass(type):
    def __new__(cls, name, bases, dct):
        dct['say_hello'] = lambda self: "Hello, world!"
        return type.__new__(cls, name, bases, dct)

class MyClass(metaclass=MyMetaclass):
    pass

my_obj = MyClass()
my_obj.say_hello() # Output: "Hello, world!"