Function
In plain terms
A reusable block of code that performs a task.
Going deeper
A function encapsulates logic that can take inputs (parameters) and return an output, called wherever needed to avoid repetition. It packages reusable behavior.
Under the hood
Pure functions avoid side effects; first-class functions can be passed around as values.
Examples
def add(a,b), reusable logic, parameters, return values, methods.
Think of it like...
Like a kitchen appliance, give it ingredients (inputs) and it produces a result.