Wrapper is an item that can contains an “Element Item” in side it. You can use it to create sections/layout/parallax background. You can find wrapper at the first tab of the page builder. Try clicking or draging the item to add it to the container area.
In live “live view mode”, blue frame represents background/sidebar wrapper item, yellow frame represents column item. Hovering over the item and you’ll see the tools bar for managing the wrapper item. You can use double click as a shortcut to edit each item as well.