The German combination of wo with a preposition is good for asking questions to clarify specific situations when warum (why) is too general. Since wo is generally translated as "where" in English, the wo + preposition combination can cause confusion among German beginners, since in this case wo is usually translated as "what." If the man is waiting (Er wartet), you may be tempted to ask what he is waiting for. Für was wartet er? would be wrong and a typical beginner's mistake — correct is Worauf wartet er? This translates as "What is he waiting for?", thus worauf is "what for."
Wo can be combined with the following prepositions: an, auf, aus, bei, durch, für, gegen, in, mit, nach, über, unter, von, vor, and zu. Note that when combining wo with a preposition that starts with a vowel, the letter r is added between wo and the preposition: woran, worauf, woraus, worin, worüber, worunter. This may seem complicated at first, but in context you will find it much easier than expected! Here are some examples from Yabla:
Der Höhepunkt des Abends, worauf alle gewartet haben...
The highlight of the evening, which everybody had been waiting for...
Caption 35, Rund um den Flughafen: Der neue Airbus A-380
Es macht viel Spaß, wobei die Füße jetzt langsam anfangen doch etwas zu schmerzen.
It's a lot of fun, although the feet now are indeed slowly starting to ache somewhat.
Captions 10-11, IAA: Traumland für Autobegeisterte
Ich weiß nicht, wovon Sie sprechen!
I don't know what you're talking about!
Caption 10, Kein Kredit: im Land der Klone
Go the the main Yabla German page and search for some of the following wo + preposition combinations: woran = on what, of which; worauf = at which, whereupon; woraus = from what, whence; wobei = whereby, although; wodurch = by which means, whereby; wofür = for what; wogegen = against what; worin = in what, wherein; womit = whereby, wherewith; wonach = after what, whereupon; worüber = about what, whereat; worunter = under which, from what; wovon = from what, whereof; wovor = of what, in front of what; wozu = what for, why.