What is exactly the event object?

I am starting to learn about event object this week, and I can not wrap my head around it.
Please, excuse any typo as english is not my native language, and I will try to be clear.

What is exactly event object? when is it executed? why do we have it?
For example:

1. when we use an anonymous function as in this example, the event object will not be renderPosts() function.

window.addEventListener(‘DOMContentLoaded’, () => renderPosts());

const renderPosts = async () => {
  let uri = 'http://localhost:3000/posts?_sort=likes&_order=desc';
  const res = await fetch(uri);
  const container = document.querySelector(".blogs");

2. when we do not use anonymous function as in this example, the event object will be renderPosts() function.

window.addEventListener(‘DOMContentLoaded’,renderPosts());

const renderPosts = async () => {
  let uri = 'http://localhost:3000/posts?_sort=likes&_order=desc';
  const res = await fetch(uri);
  const container = document.querySelector(".blogs");

I do not see the difference and not really understand it. I have been searching about event object, but still confused.

Appreciate any explanation.