This CSS code is styling the background of the whole HTML document. Here is what each part of the code does: 1. `background: url('background-image.jpg') no-repeat center center fixed;` - This line sets the background image of the HTML document to 'background-image.jpg'. - The `no-repeat` value ensures that the background image is not repeated. - `center center` centers the background image both horizontally and vertically. - `fixed` ensures that the background image stays fixed when the page is scrolled. 2. `-webkit-background-size: cover;` - This line sets the size of the background image to cover the entire background in Webkit browsers (like Chrome and Safari). 3. `-moz-background-size: cover;` - This line sets the size of the background image to cover the entire background in Mozilla browsers (like Firefox). 4. `-o-background-size: cover;` - This line sets the size of the background image to cover the entire background in Opera browser. 5. `background-size: cover;` - This line sets the size of the background image to cover the entire background in standard CSS. Overall, this code ensures that the background image covers the entire background of the HTML document and remains fixed as the page is scrolled. The different vendor prefixes are used to ensure cross-browser compatibility for setting the background image size.