This page demonstrates CGI functionality. When you click on the POST button, HTTP POST request is sent to the server, CGI function is invoked. As a result any text written (80 characters max.) in the input field is stored in memory. After that, response in sent back to the client, containing information for page refreshing.
Similarly, when you click on the GET button, HTTP GET request is sent to the server and then processed in CGI. As a result content of following text field is replaced by text stored previously by POST request.