This weight number will allow the site admin to determine where in the node the widget will appear in relation to the content. The weight number is a commonly used practice among all drupal modules to determine order of placement of objects vertically on the page.
The default weight of the plus 1 minus 1 widget places the it at the bottom of the page. With this option administrators can place it at the top, the bottom or sandwiched in between other pieces of content in the node.