Of this, that or it.
"The contract specifies a penalty for late payment, and interest on thereof shall accrue daily until settlement."
In plain English: Thereof means something belongs to or comes from that thing mentioned earlier.
"The lawyer explained that the client would receive half of the proceeds thereof."
Thereof comes from the combination of the words "there" and "of," which has been used in English since at least the Middle English period. It originally functioned as a phrase meaning "from that place or thing."