In a nutshell:
ENY is some kind of "value" of a plane. Every plane has a ENY value (check this in hangar). The lower ENY, the more "capable" a plane is considered to be.
Whenever one side gets a bigger numerical advantage, ENY limiter kicks in, reducing the availability of certain planes.
You can check current ENY restrictions by rightclicking on the clipboard map, then select "country status"
And by the way, you get more perk points (need them to fly planes like Tempest or 262), if you fly a plane with a high ENY value and shoot down planes with a low ENY.
Shooting down a 109F in your N1K gives you about 0.17 perks, killing a N1K in a 109 produces about 6 perk points