In other words, how large and diverse is the scope? Most commonly, this refers to the question of whether or not the reaction tolerates a variety of functional groups (with different stereochemistries, if your being picky) adjacent to the position that the chemistry is acting upon. Also, is the reaction tolerant to various other functional groups elsewhere in the molecule (ie. won't mess with other parts of the molecule). It may also be important to consider if the reaction works under a wide variety of solvents/temperatures/concentrations.