Are Fannie Mae MBS guaranteed?

Could you please clarify if Fannie Mae MBS, or mortgage-backed securities issued by Fannie Mae, are indeed guaranteed in any way? I understand that Fannie Mae is a government-sponsored enterprise, but does this mean that their MBS offer some form of protection or security for investors? Are there specific terms or conditions that need to be met for this guarantee to be in effect? And, if so, what happens in cases where the underlying mortgages default? Thank you for your insight.