What is auth-nxdomain?

[Agustina Buccella]

I read the bind 9 Administrator Reference Manual and said that the 
auth-nxdomain is:
auth-nxdomain If yes, then the AA bit is always set on
           NXDOMAIN responses, even if the server is
           not actually authoritative. The default is no;
           this is a change from BIND 8. If you are using
           very old DNS software, you may need to set it
          to yes.

but I don't understand very well? Can somebody explain me in english?? jaja

Thanks
Agustina