'von' should be translated as 'of' or 'from' and indicates where the (noble) family was originally from. 'Zu' was sometimes added to show where the family resided at that time, so you could be 'von' one place, but currently living 'zu' somewhere else. A combination of 'von' and 'zu' (as in Mr von und zu Guttenberg's case) thus shows that when the special privileges of the nobility in Germany were abolished in 1919 and titles were converted to plain old surnames, the family still resided at their original ancestral home.