|Figure 1: Elhaym, Maiden of Light wishes to engage Damaris!|
Basically it allows a unit to ignore movement penalties for terrain and move through/over other models without impediment. It also means they cannot be Intercepted when Charging except by other Flying units!
Figure 1: Elhaym, Maiden of Light wants to Attack Damaris. She has 4 AP left, so she could Charge (ignoring the Hellhound as she moves over it) using her Flight, or could Move (1 AP; again ignoring the Hellhound) then use Wings of Withdrawal to Attack and hopefully move again. Ether way the Hellhound cannot Intercept, since it does not have Flight.