These are abnormally enlarged veins in the lower part of the esophagus. They can develop when normal blood flow to the liver is blocked. When blood get 'backed up' it backs into smaller, more fragile blood vessels in the esophagus. It can also back up into smaller vessel in the stomach or rectum and cause those vessels to swell as well.
The varices don't produce symptoms for the most part unless they rupture and bleed. If that starts, it can be a life-threatening condition.