Microsoft doesn't provide a telnet program with its current operating systems by default, but a commonly used program for telnet and SSH on Microsoft Windows systems is PuTTY which is free. A telnet client is provided with many Linux distributions and one is available with Apple's OS X operating system. PuTTY is also available for Unix/Linux systems. E.g., see How To Install & use Putty in Ubuntu Linux written by Pradeep Kumar on July 13, 2014.
With a telnet program you can send commands/headers to a web server to emulate those that a browser would send to a web server and observe the responses from the server. This can be useful in some troubleshooting efforts. E.g, see Testing acces to a website using PuTTY for how to configure PuTTY on a Microsoft Windows system to perform a basic connectivity test to port 80 and request a web page for a particular site from the server.
