Metaclasses allow customizing class creation in Python. They are classes that create other classes. When a class is defined, Python calls the metaclass' __new__() method to create the class object. The metaclass' __init__() method then initializes the class. Common uses of metaclasses include making classes immutable or decorating class methods. For example, a metaclass could add logging to method calls. Metaclasses provide a powerful but complex way to customize class creation in Python.