Your send string means that you are requesting the root of your http server (or /), and that you are willing to talk to it with the specified http version (1.1). And because the host header is mandatory when using http 1.1, you must send a host header. While this example is sending an empty header (just to comply with the http 1.1 rfc), you may sometimes specify the correct hostname of your application to be sure you are getting the correct answer from the server. Monitors don't need to keep the connection alive, so you also tell the server to close it immediately.
Regarding the receive string, because it is empty, any response from the http server will be considered as a valid answer and the member marked up. You may want to parse the response and make sure an exact string exists within it before marking the server up, for this you specify the string to match in that field.
Examples for receive string can be any text within the html (e.g. Welcome to my awesome server) or within the status or the headers (e.g. 200 OK)