Stiffness is measure of the rigidity (as opposed to flexible) of an object or tissue such as the vocal folds. Technically it is the amount of force required to displace or move a tissue. The stiffness of an object  can be measured by how much force you would need to exert to indent the object. Clearly it requires more force  (measure in Newtons) to indent (measured in millimetres) a tennis ball say 5mm than one made of foam, and it would be impossible to generate enough force to indent a golf ball. The golf ball is therefore the stiffest requiring a massive force to indent it by 5mm compared to a tennis ball and the foam ball (least stiff). If the vocal folds are tensed or scarred then more force will be required toget them to vibrate because they are stiffer.  (Units of measure: Newtons per metre).