You met these in the introductory tutorial and there is not much more to learn. A div element is like a container, a box. It is used to hold other things.
Open media.html in your editor. Also view it in a browser. Fairly close to the top of the code you should have two images (img elements). Before the second one add <div> and after it add </div>. You now have two images:
Save and view the page and the images should now be on different lines.
In-line and block
Images are known as in-line elements. They will be displayed one after another in a line. Paragraphs and divs are block elements. They get a new line. Paragraphs and headings are also block elements so they get their own line as well.
A block is an area of the page which by default takes the full width of the page. An in-line element uses only as much space as it needs.
For now just remember this as it will come back later.