How is the function in my eventlistener able to receive an event without using function parameters?

Question

I'm working on a very simple game using vanilla JavaScript and the behavior I'm seeing right now goes against everything I've learned about with functions. I don't understand why getInput can be called without the usual parenthesis that you'd use when calling functions. In fact, when I add parenthesis like this: document.addEventListener("keydown", getInput());, it doesn't work at all.

So then, how am I able to receive and print the event key without it having been passed as a parameter?

document.addEventListener("keydown", getInput);

function getInput() {
    console.log(event.key);
}