If you just press enter, without a command, you’ve issued no command.
Nothing happens (you haven’t told the computer to do anything), so the prompt appears again.
What actually happened there is you submitted a command line, and the computer parsed it (which is the first stage of interpreting it), and found nothing there. So immediately it has finished. When it’s finished, it throws up a new prompt.
Press enter enough times and you’ll see the output being pushed up the screen as the prompt makes a new line each time.