Content element shows the content created for page. You can edit default content of page directly in editor included in this element while you edit layout of page in Page Builder, or you can edit page content in standard WordPress way, through default WordPress Edit page.
From WordPress version 5.0 where Gutenberg editor is used for Pages and Posts, Content Element doesn’t show the Gutenberg editor directly in the Page Builder, you can see the button which move you to Edit Page of page currently edited in Page Builder.