An iterator is a programming construct that allows you to traverse through a collection of items, such as lists or arrays, one element at a time. It provides a way to access each item without needing to know the underlying structure of the collection. This makes it easier to work with different types of data structures in a consistent manner. In many programming languages, iterators are implemented using methods like `next()` to retrieve the next item and `hasNext()` to check if more items are available. Common examples of collections that use iterators include arrays, lists, and sets. This functionality is essential for tasks like looping through data and processing elements efficiently.