Things things are happening here. First, in Dutch, one cannot use a preposition and it / that / this / those / these / which. Instead, these become here, there, and where. These then combine. Instead of van het, van dit, van dat, and van wat, one says ervan, hiervan, daarvan, and waarvan. This is like early modern English (think Shakespeare): therefrom, herefrom, wherefrom.

Second, in Dutch, these altered-reversed-combined prepositional phrases are often split with adverbs, etc. This is sometimes optional but preferred, e.g. Ik lees daar een hoofdstuk van, is preferred and Ik lees een hoofdstuk daarvan is acceptable. In your example, Ik weet niet waar je praten over, is preferred, but I think Ik weet niet overwaar je praten, is acceptable.