Okay kiddo, did you ever use walkie-talkies with your friends before? If you did, you know that you had to hold a button down to talk and then release it when you were done. That button you pressed is like an endpoint in communication.
In more grown-up terms, an endpoint is where information starts or ends its journey during communication between computers, phones or other devices. It's like a mailbox that sends and receives information. So, for example, think of your phone as an endpoint for communication. When you send a text message, your phone is an endpoint that sends it to another phone or device, which is the endpoint that receives the message.
Overall, an endpoint allows devices to communicate with one another and exchange information.