GET method is used when the page is loaded from server to your browser. If it works, your web page is available to load. It is the most common method among all, when you type a site address in your broswer - this method is used to show you the site content.
HEAD method is similar with GET, but it load from server only headers of your page. It is useful if You want to test just accessibility of a web page or review its headers.
POST method is used to send information from you to server. If a page includes text field(s) for filling, POST method is used when you click button "submit" (or whatever is name of that button). It brings information you have typed into those fields to server. So, if there are some text fields on your page, you can use this method. If it works, your web page is available to use, and text, typed into fields on your page, is able to be transferred to the server. An example of using POST method for Website Monitoring is provided in this article (You provide specific keywords, which are inserted into text fields).