The FileReader is a built-in JavaScript object that allows web applications to read the contents of files stored on a user's computer. It is commonly used in conjunction with the File and Blob objects, enabling developers to access file data from input elements or drag-and-drop interfaces. The FileReader can read files as text, binary data, or data URLs, making it versatile for various applications. When using the FileReader, developers can listen for events such as load, error, and loadend to handle the file reading process. This functionality is essential for tasks like previewing images, processing text files, or uploading data to servers, enhancing user interaction with web applications.