ChatGPT is a chat program. You write, and it writes back. People use it for text, code, translation, and a plain explanation. They search its name a lot this year because it stands on its own, both on the web and on the phone.
OpenAI makes it. There is a free side and a paid side. Which model is open, and how many questions you can ask in a day, changes from time to time. The name on a button may not be the same tomorrow.
It does not open a file on your computer by itself. It sees the text you paste or upload yourself. A password, a card number, and a private key should not go there.
The answer is not always right. A number, a date, a law, and a piece of code can come out wrong. It can sound sure and still be wrong. Check an important fact somewhere else. Do not accept code before you run it.