What is @id as passed to $resource?

$resource("/entries/:id", {id: "@id"}, {update: {method: "PUT"}})

What is @id?

On the $resource doc page someone says this below, but I still don't understand.

If the parameter value is prefixed with @ then the value of that parameter is extracted from the data object (useful for non-GET operations)." The data object here refers to the postDataobject if non-GET "class" action is used, or the instance itself if non-GET instance action is used.