Fiddler is a web debugging proxy tool that allows developers to monitor, inspect and edit HTTP and HTTPS traffic between a client and a server. It provides a user-friendly interface that displays all the HTTP requests and responses exchanged between the client and the server, including the request and response headers, body, and timings. Fiddler also offers powerful features like breakpoints, autoresponders, and filters, that enable developers to troubleshoot and test web applications more efficiently.