What is a normal gastroesophageal junction?

Can you please explain in simple terms what exactly is a normal gastroesophageal junction? I've heard it mentioned in relation to acid reflux and other digestive issues, but I'm not quite sure what it is or how it functions normally in the body. Is it something that can be affected by certain lifestyle choices or medical conditions? I'm eager to learn more about this crucial anatomical structure and its role in maintaining good digestive health.