The shortcode name is the part just after the left square bracket. Create a shortcode using the wizard to see how to specify attributes. The two attribute textareas below are for attributes that specify one respectively multiple post IDs.
On each line, specify one attribute.
The standard delimiter for multiple post IDs is a comma. If the shortcode uses something other than comma between the IDs, write the delimiters after the attribute. Like so: ids ; : ,