Perks = Points
"Perks" are points you earn by, well, killing stuff. Once you earn enough "Perks" you can fly planes called "Perk Planes". Depending on how much you have saved and how much the plane costs, you can fly that plane. If you crash it or get killed, you loose the perks. The value of the "perks" varies to the player count on each country. So, you might take up a "perk plane" for say...41 perks. When you land and go into the hanger, the cost may now be 47. The cause of this is more people signing on to a different country.
Hope this helped
-SX