In YAML this is a tag, a tag is used to associate meta-data with a node.  So in this case we're using a tag named "include" to denote that the following value is a filename to be included (this is not a standard YAML include, but its been used by other applications, and it falls within the application local tag name space).  If we want includes to happen at locations other than the root, this is more or less the way to go...  Alternately we could add a prefix to values that are supposed to be used as config files:

mapping: @@this_is_the_name_of_an_include_file