"clone" at active_record relation

inside Rails source, at
I found a lot of calls to:

    relation = clone

I can understand where id the "relation" call, but... how about "clone"
Perhaps "clone" comes from Ruby1.9-Kernel, but I am not sure.
And github color it in blue, like a class...weird...

    def where(opts, *rest)
      relation = clone

      if opts.present? && value = build_where(opts, rest)
        relation.where_values += Array.wrap(value)


Can somebody explain this "clone" ? Where it come from ?