The most important action of Secretin is to
Main Action of Secretin:
Secretin increases the secretion of bicarbonate by the duct cells of the pancreas and biliary tract. It thus causes the secretion of a watery, alkaline pancreatic juice (with high content of bicarbonate). Its action on pancreatic duct cells is mediated via cAMP.
Secretin causes alkaline pancreatic juice to flood into the duodenum, neutralizing the acid from the stomach and thus inhibiting further secretion of the hormone - feedback control.
Other actions:

Inhibits gastric juice secretion.
Inhibits gastric motility.
Causes contraction of pyloric sphincter.
Augments the potency of action of cholecystokinin on pancreatic secretion.
neutralize the acid from the stomach
increase secretion of bicarbonates by pancreas
decrease gastric secretion
cause contraction of pyloric sphincter