My mother has developed esophagus muscle dysfunction. What is it exactly?

Swallowing trouble. The muscles that wrap around the esophagus, as well as the sphincters (both upper & lower) coordinate their contractions to produce peristalsis, thereby propelling food from the back of your mouth to the stomach, & keeping that food from backing up again. There are a lot of diseases that affect the esophagus muscles--some minor (presbyesophagus) & major (like achalasia, parkinson's, scleroderma).