It is name of any Navy aircraft which carries the President of USA
However, president don't often fly on Navy Aircraft.
Only once was this designation used.
The Lockheed S-3 Viking aircraft used to transport the
George W Bush to an aircraft carrier for his Mission Accomplishment
speech. This plane is now housed at National Naval Aviation Museum
at NAS Pensacola in Florida.
Based on information from Wikipedia.
https://en.wikipedia.org/wiki/Navy_One
His normal plane is Air Force One.
His normal helicopter is Marine One.