Also known as the hepatic portal vein, this structure is a deviation from the standard mindset of "all blood that flows away from an end organ flows towards the heart".
The portal vein is formed at the confluence of the superior mesenteric vein and the splenic vein. It carries blood from most of the gastro-intestinal tract (from the stomach down to the rectum), the pancreas and the spleen to the liver. In the liver, blood from the portal system flows in different tracts to tributaries of the hepatic artery and intra-hepatic bile ducts.
The portal vein carries absorbed nutrients from the gut to the liver to be processed and stored. Broken down carbohydrates and proteins are carried in the portal vein but not broken down fats. Lipids are carried away from the gut by the lymphatic system and are dumped in the left subclavian vein by the thoracic duct. Why is this significant? Because extra lipids in your circulation add to your risk of atherosclerosis ... ask any anatomist, if they were to redesign the digestive tract, they would make it so that lipids were preprocessed by the liver before being released into the circulation rather than having lipids released into the circulation and only then being processed by the liver.
(Relatively) common problems with the portal vein are portal hypertension and portal vein thrombosis.