Communication between Arduino and PC

I don't understand the last sentence.

Think about how a telephone works. If you want to call someone, and ask a question, you don't dial the number and start speaking right away, then hang up when you don't get a response.

You dial the number ( open the serial connection) and wait for the person to answer and say hello. Then, you ask your question or make your statement.

You are not currently waiting for the Arduino to say hello.