The structure indicated is the pancreatic duct (duct of Wirsung) of the pancreas.
The pancreas is an organ which lies posteriorly to the stomach and extends from the duodenum to the spleen.
The pancreatic duct extends from the tail of the pancreas through the body of the pancreas into the head of the pancreas where it joins the common bile duct to form the hepatopancreatic ampulla, also known as the ampulla of Vater, which empties into the descending duodenum at the major duodenal papilla.
In addition to the main pancreatic duct, there is also an accessory pancreatic duct (duct of Santorini), which empties into the minor duodenal papilla.
Obstruction of the pancreatic duct, commonly due to gallstones can lead to acute pancreatitis.
Learn more about the basic anatomy of the organs of the digestive system in this tutorial.