Flyover basically gives you a 3D representation of the city your looking at. Apple launched with only a handful of cities including Sydney and Melbourne here in Australia but I'm happy to say that it looks like they quickly expanding as my home city of Perth has now been added.
To check out flyover mode (or see if your city has been included) make sure that you are on either Hybrid or Satellite view (You can toggle these by tapping the i at the bottom right of the maps screen) and then zoom in until you see the Buildings icon at the bottom of the screen, tap it and your away! Swiping up or down with 2 fingers will give you a different perspective and using to fingers in a twisting motion will let you rotate the city or buildings your looking at in 3D.
I'm about 10 kms out from the city and could get a great 3D rendering of my place.
Go check it out and see if your city has been added.
Here was Scott Forstall introducing Apple Maps in 2012: