ChatGPT is a modified version of another AI language model, called GPT, made by OpenAI.

While the exact technical details aren't available, we do know it's a complex language model trained on a vast data set of written word. There is also some component to the training that teaches ChatGPT what appropriate responses to prompts look like, so that it can work in a chatroom context.

In more ELI5 terms, it's a program that was designed and trained to learn how language works and how to respond to prompts.

When you send it a message, it does a large, complex calculation to make a good response to your message.