The Divi Builder comes pre-installed in the Divi theme. It is a visual drag-and-drop builder that allows you to create your own custom page and post layouts and designs. If you're using the Divi theme, you'll automatically have use of the Divi Builder. They also make the Divi Builder available as a plugin so you can use it with other themes as well. By using it as a plugin, you'll have the page builder capabilities with your existing theme.