Normally lower end of esophagus where LES is present helps in prevention of reflux of food from stomach to esophagus. So basically LES acts like a valve and helping in antegrade movement of food. That's why after eating food when it enters stomach, LES prevents it from getting it into esophagus again. So LES acts as a gatekeeper and allows one way movement. If tone is reduced it means it is weak and may cause reflux of food from stomach to esophagus which can cause heartburn, acidity and chest pain