It's called a dual functioning organ because it can act either as an endocrine organ or as an exocrine organ. Both of which have different functions.
Explanation:As an ENDOCRINE organ it secrets HORMONES that play a role in the control of body metabolites particularly glucose. An example of these hormones would be isulin(which decreases the level of blood glucose by permitting entrance of glucose into cells in the liver and adipose tissue), glucagon(which increases blood glucose by permitting glycogenolysis), and somatostatin which inhibits both insulin and glucagon.
On the other hand as an EXOCRINE organ, it releases ENZYMES that help in the degradation of the food we eat for more efficient utilization by our cells. To name a few are lipase and amylase.