“What is an endoscopy?”

This is a procedure to look into the esophagus, stomach and duodenum. It's also called an EGD. You are sedated for this so it is not painful.
Here is a reasonable explanation: https://www.verywellhealth.com/upper-endoscopy-p2-1741797

An endoscope is a long, flexible tube with a light on the end. It can be used to look into the stomach (upper endoscopy), colon (colonoscopy), sinuses (sinus endoscopy), joints (arthroscopy), etc. Most of these procedures are done with the patient under light anesthesia--so usually there is very little or no pain when done by a very experienced and sophisticated doctor and medical team.

Most of the time, people use the word "endoscopy" to refer to an upper endoscopy, which is putting a flexible tube down into the stomach. The endoscope is thin and flexible, with fiber optics that allow a good view of the "insides" of the stomach, etc. It is completely painless. Most of the time, you will not recall the procedure because of the sedative that they use. They might use this type of procedure to check for ulcers and other problems with the gastrointestinal tract. It is very common and it is not a big deal.