A group of geese flying in the air is called either a skein or a wedge. A skein is a group of geese in flight, and a wedge is when they fly specifically in a "V" formation.
Geese fly in a "V" shape because it improves their efficiency. As the geese in front flap their wings, they push pockets of air up and down. The geese near the back conserve their energy by riding the air that's being pushed up. They don't have to flap their wings as much and can fly farther at a steady pace.