The `reduce()` function is a method in programming, commonly found in languages like Python and JavaScript. It processes a list or collection of items, applying a specified function to combine them into a single output value. This is done by iteratively taking the result of the function and the next item in the collection until all items have been processed. Typically, `reduce()` takes two arguments: a function and an iterable. The function defines how to combine the items, while the iterable is the collection being processed. This method is useful for tasks like summing numbers, concatenating strings, or finding the maximum value in a list.