According to the page you linked to it is an experimental value for defining that the elements will be replaced by their pseudo-box and their child boxes.
It will be used to ignore a grouping element. Only the children within that grouping element will be considered instead.
In effect it’s like none, but only for that one element. None and contents differ only by how their children are affected. None doesn’t display the element as well as none of its children. Contents however just doesn’t display the element, with the children being unaffected.