What happened? Python "printed out" Hello, world! for you. It's an old programming tradition, going back to 1970s. It is a traditional first command executed by people who are learning a given programming language, which illustrates the basic syntax of the language.
In Python one can simply execute print("Hello, world!"). In other programming languages it might not be that simple! Feel free to search for examples of a "Hello world" code in C++, Java, HTML, JavaScript or other languages.