A URL parameter, also known as a query string or query parameter, is a set of characters that follows the question mark in a URL and is used to pass information to a web server. These parameters are typically used to customize the content or functionality of a webpage, such as filtering search results or tracking user preferences. They consist of a key-value pair, separated by an equal sign, and can be added to the end of a URL or inserted within the URL path. URL parameters allow for dynamic and personalized web experiences, making them a crucial component of modern web development.