The reduce() function is a method in programming, particularly in languages like JavaScript and Python, that processes a list or collection of items to produce a single cumulative result. It works by applying a specified function to each element of the collection, carrying forward the result to the next element until all items have been processed. Typically, reduce() takes two arguments: a function and an initial value. The function defines how to combine the elements, while the initial value serves as the starting point for the accumulation. This makes reduce() useful for tasks like summing numbers, concatenating strings, or aggregating data.