Try clicking on that now, and another window will appear in which you will see the HTML code which was written to make the page look and behave as it does. This allows you to edit the page, but you're not editing the actual page itself, just your own copy of it. That does mean that you can save it easily if you want. Netscape displays tags in a different colour to make them stand out. When Netscape or Explorer display a page normally, the tags are not shown in the normal window, only in the window to view the source.
Tags are like instructions to the browser software.
That's why it's a good idea, just in case, to bookmark the page. The next thing to notice and remember is that when you change the size of the browser window, the text is adjusted so that it fits into the window. This is completely independent of the HTML that you write. If it is full-screen, click on that button next to the X in the top right of the window, and the window will shrink to cover only part of the screen. Then change the size of the window by dragging one of the sides or corners.