"Thereof" means "of that" or "of it", pointing back to something already named. It comes AFTER the noun it belongs to: "the house and the contents thereof" = the contents of the house. "He read the report and questioned the accuracy thereof." It is a formal, legal-sounding word, and its whole job is to avoid repeating a long name — instead of "the accuracy of the Nineteenth Annual Environmental Impact Report", you write "the accuracy thereof". You will find it in contracts, laws, wills, technical writing, and the Bible. In everyday English people just say "of it", "its", or "of them". One phrase escaped into normal speech and you should know it: "or some variation thereof", meaning "or something like that" — "He said he was tired, or some variation thereof." People use that one half-jokingly, enjoying the stiff sound.