When you click on the 3D view button, the page goes into 3D view mode; in this mode, you can see your page presented in a 3D view in which nested blocks of HTML are increasingly "tall," projecting outward from the bottom of the page. This view makes it easy to visualize the nesting of your content.
By clicking and dragging the view, you can rotate and re-orient the 3D presentation of the DOM hierarchy of your page to see it from different angles, to better examine its structure. Off-screen elements become visible, so that you can see where your elements are located in relation to the visible content. You can click on elements to see their HTML in the HTML panel or to update the breadcrumb trail.