The divider is used to create spacing between fields. Use the divider field to make a field group look less cluttered. The divider can be a divider with a line (as pictured below) or just plain white space.
Just add the field in the field group admin and you will see something similar to the image above when you edit your post.
<?php // array to be used as an argument to simple_fields_register_field_group: array( 'name' => '', 'slug' => "divider_1", "type" => "divider", "options" => array( "appearance" => "line" // line | white_space ) )