This is a  procedure of determining  the potential if any for improving each  of the non basic variables in terms of the objective function. To determine  this  potential  each of the non basic variables ( empty  cells) is considered one by one. For  each such cell well  find out as to what effect  on the total cost would  be if one unit is assigned to this  cell. Within  this information  then we come  to known  whether  the solution is optimal or not. If not we improve that solution. To understand  it we refer to the example  containing the initial  basic  feasible solution  obtained by  least  cost rule.