Basically the amount of aircraft that can pass over a certain point during a set period of time. This is laid down by either the country of overflight or maybe nowadays by the CFMU. These regulations can change at any time due to the airspace complexity and density of traffic. There are also planning stages for the regulations and reactive stages to implement them on a daily basis. Now the single european sky is slowly being implemented and other complicated issues around europe they will constantly change. Hope this helps its a while since I researched this for my own advancement.
Rgds.
PS Also more complicated than I have explained.