I think he is referring to a vBulletin feature where members can be thanked for so many referrals they have from people that they have brought to the board. Then on the forum summary it tells you who has brought the most members to the board by saying 'Highest Referral Total: Member Name(Amount)'.
It's also an oppurtunity to see who is helping bring activity to the board so you can help reward them in some way..