I can't really understand what accuracy means, I know that proxymity is a range of the iBeacon (immediate, near, far, unknown) based on the strenght of the signal, but what about accuracy?
The doc says
The accuracy of the proximity value, measured in meters from the beacon
So does it mean that it's a value that says how much meters you're away from the iBeacon or it's just a value that tells you if you're close to one of the proximity zones?
Accuracy is an estimate of the distance in meters to the beacon. This is only a very rough estimate based on Bluetooth signal strength (RSSI) and varies quite a bit due to radio noise.
Due to the error, Apple recommends it be used only to determine the relative positions of beacons when multiple are visible.
You can read more about how this works here: http://developer.radiusnetworks.com/2014/12/04/fundamentals-of-beacon-ranging.html