def creates a function. Without a return, it returns None.

def greet(name, greeting="Hi"):
    return f"{greeting}, {name}"

greet("Ada")
greet("Ada", greeting="Hello")

• A parameter with = has a default and may be left out.
• *args collects extra positional arguments into a tuple.
• Anything after *args can only be passed by name. Those are keyword-only parameters.
• **kwargs collects extra named arguments into a dict.

def log(*parts, sep=" "):
    return sep.join(parts)

log("a", "b")           # "a b"
log("a", "b", sep="-")  # "a-b"

Names assigned inside a function are local to it. A default value is computed once, when def runs, so never use a list or dict as a default. Use None and create the list inside.

Write clamp(value, low=0, high=100) limiting value to that range. Write average(*numbers, default=0) returning the mean of its arguments, or default when there are none.

The explanation opens here once every check passes.