When you switch on member ratings, members will have a number in their profile indicating their value to the forum. This number is determined by a formula which you create at Admin Panel -> Members -> Settings. You can include any database values in the formula in the form of $this->fieldname, and any member functions of the class (from classes/members.php) in the form $this->functionname().

The member rating will normally update on the submission of a link or comment by the member. You can also regenerate members to update it.