waar ... op/waarop is called a pronominal adverb. It replaces a preposition and a pronoun, e.g. wat + op, by turning the pronoun into a locative adverb ("waar" always replaces "what") and the preposition into a prepositional adverb. Here, the preposition doesn't change, but for things like "met", there is a change: "met" becomes "mee" (with what -> met wat -> waarmee)
op + wat becomes waarop
These are much more common in Dutch than in English.
Some examples of this in English are:
by this becomes hereby
in that becomes therein
It is very common for these pronominal adverbs to become separated in a sentence. This is why the sentence begins with "waar". It belongs with "op".