This template is used to quickly re-use a named reference. This works with references named using any method, be that a Cite template, or using ref tags normally.
The syntax is:
You must name a reference before inserting this template or it will trigger an error message.
If you want to refer to a named reference that is part of a group, you can use the second parameter:
See here for more on reference groups.
If you prefer, you can use named parameters instead - to make it clearer which parameter is which, etc.