A URL decoder is a program or function that takes a URL-encoded string as input and returns the decoded string. URL encoding, also known as percent encoding, is a process of encoding special characters in a URL so that they can be safely transmitted over the internet. The URL decoder reverses the encoding process by converting the encoded characters back into their original form. The decoder is commonly used to parse URLs that have been transmitted over a network or stored in a text-based format. The decoder can be implemented in various programming languages and is an essential part of the process of retrieving information from a URL.