Theme elements can be accessed only when you creating or modifying Theme Templates. These all dynamic elements and the content of specific element depending on page type. With Theme Elements you can design single and archive templates. Some elements like Breadcrumbs or Page Title can be applied for both simple and archive type of a Theme Template. But some elements must be added for a specific template type. For single types you need to have a Post Content element and for archive type the Posts Loop element must be presented.
For WooCommerce there are additional theme elements like Product Images, Tabs and etc. So you can design a single product page as you wish simply by dragging and dropping theme elements.
Check Official Documentation for Theme Elements