When do I need a () at the end of a function?

Question

Long time listener, first time asker.

I'm following a tutorial and we have the following code:

const closeModal = function () {
  modal.classList.add('hidden');
  overlay.classList.add('hidden');
};

overlay.addEventListener('click', closeModal);

document.addEventListener('keydown', function (e) {
  console.log(e.key);
  if (e.key === 'Escape' && !modal.classList.contains('hidden')) {
    **closeModal();**
  }
});

My question is when do I need a ( ) to explicitly call a function? In the first case I don't need it, but in the second case I do. I tried running it without the ( ) but it didn't work.

Thanks for the help and any terminology would be useful. I tried google searches but didn't know the right keywords to use.