It's my understanding that "volere dire" is an idiom that's usually translated into English as "to mean", as in this example.
I'll go out on a limb and say that French and Spanish have a similar idiom, if you know what I mean.
In this context, the closest equivalent is 'that which' - 'you don't understand that which I want to say' (or, less literally, 'you don't understand the thing that I mean'); in English, we frequently omit the 'that' and just say 'what', which is how the answer is translated. There's an interesting discussion at http://forum.wordreference.com/showthread.php?t=226708. I've also heard somewhere that 'quello che' is much more common in speech, while 'cio che' is more common in written Italian (generally considered essentially interchangeable).