In a nutshell: code that manipulates code.

Common examples:

• Decorators
• Metaclasses
• Descriptors

Closures: You can make and return functions.

def make_adder(x, y):
    def add():
        return x + y
    return add

Reference: https://www.ibm.com/developerworks/library/ba-metaprogramming-python/index.html