Goal

Console must log a message only on clicking the button.

Set-up

const button_l = document.querySelector('.btn-left');

method-1 (Using arrow function as a callback)

button_l.addEventListener('click', () => {
    console.log('I am in')
});

method-2 (Using function as a callback)

button_l.addEventListener('click', changeSlide());

function changeSlide() {
    console.log('Am I globally present?');
}

Problem Facing

1. Using method-1 gives the desired results.
2. Using method-2, the function is called on page load, but when I remove the parentheses (from changeSlide() to changeSlide), it functions as it should, why is it so?

I think it is something very fundamentally obvious that I do not know.

I appreciate a link/reference to any helpful theory/understanding.