In one sentence
ChatGPT is a conversational robot powered by an LLM (a "large language model") that guesses what to reply to you word by word, like an autocomplete on steroids that has read a good part of the Internet.
🎰
The analogy that works
Imagine a mental slot machine that, for each word, calculates thousands of possibilities and picks the most likely one. Except this machine has read billions of pages, and is incredibly good at guessing what sounds right. That's ChatGPT.
(EN content abbreviated — full FR version is the primary content)