Hi,
This table is what is registering the "helpful" / "not helpful" votes on votes.
The column vote_user_id in that table correspond to the vote_id column in the vote table (so that the system know which vote was noted as helpful/not helpful), and the column vote_user_user_id is linked to the user_id of the user column (and in that case it requires the user to be logged in to not be 0).
I would recommend to turn on the "Users must be registered to note other comments" setting in the HikaShop configuration so that anonymous helpful/not helpful votes cannot be done.