First of all its poison, which is dangerous to leave around in a house that you are selling.
A curious child or a adult wondering what is in the jar could be poisoned.
Then written on the side of the jar is that this (unidentified) poison should be poured into the vertical clean-out on the sewer twice a month to keep them free, along with a comment about wearing rubber gloves.
Considering that the disclosure did not mention any sewer drainage issues, we recommended our client to get the drain inspected with camera scope. (We hear back from our happy client that the drains were cracked and partially blocked with tree roots).