Tag would be the name of the tag you want to style and name, which is optional, would would be whatever you called that particular style.
For example, because the <body> tag defines the content of your web page, you could use the tag body with no name which would apply to all of the pages content.
Your property would be whatever characteristic of the tag you would want to change which takes the value and applies it. E.g. you could change the background-color of the page with ' background-color: #000000 '.