ENY is a few things in game... basic terms is the ENY value of the plane is how "good" it is or how easy to get kills in.
notice the la7 has an ENY or 5
notice the C.202 has an ENY of 60
now this value comes into play in many ways.
first off is perk points. above someone gave a good example. if you are in a plane with an ENY of 20 and shoot down a plane with an ENY of 10, AND land the kill, you will get 2 perks standard. on the reverse, you in the ENY 10 plane killing the ENY 20 plane, you get .5 perks.
the ENY value also is used for "country balancing" when one country has too many people, there is a formula that figures which planes to limit. if that formula states that ENY limiter of 9 is in effect, planes with an ENY value of 9 or below will not be allowed. in theroy this is to stop the country with big numbers from flying all the "best" planes, basically make it "easier" for the smaller country.
there is also a perk multiplier that is effected by country numbers also, but that is seperate from the ENY values, and stuff.
the perk multiplier works in the way that if the multiplier is 2, in the above situation instead of getting 2 perks you'd get 4. or in converse, if the perk multiplier is .5 instead of 2 perks you'd get 1.
these are all in place to help motivate people to balance the numbers in the arena.
but...
the ENY value of the plane is just a value. like i said people call the ENY side balancing thing the ENY, AND the planes value ENY.
hope this cleared it all up.