What does YF stand for?
The 'Y' in YF stands for prototype according to the Tri-Service aircraft designation system. The 'F' stands for fighter, so YF stands for prototype-Fighter. These aircraft are operated by the US Department of Defense (USAF or USN).

For example, YF-22 is the prototype (technology demonstrator) version of the F-22 Raptor.

The 'X' series is the name given for strictly experimental aircraft; i.e. they are not prototypes.