Visual Composer is used for editing of pages in Crane theme. It enables creation of web pages of any complexity with the help of Drag-n-Drop, without necessity to write HTML code.
Visual Composer can work in any of three modes: Frontend editor, Backend editor and Classic mode.
Frontend editor enables to edit site pages online, i.e. you can see how changes will look on website pages at once. Backend editor displays sketchy page structure in the form of block and elements hierarchy. In Classic mode, the page content is displayed as text, in the form of macros called shortcode in WordPress. You can read more about shortcodes in WordPress documentation.