Headphone question

[Greenlizard0]

What does impedance mean when looking at the specs for headphones?

I have looked at several definitions that either contradict each other or just (for me anyway) don't make any sense?

 

[Greenlizard0]

So the higher the Impedance the harder to drive, therefore more voltage/current is required to "get the best out of" a pair of headphones.....is that it in a nutshell?