Can somebody explain why -1 is used in javascript

I am watching a javascript tutorial to create testimonial slider.in this javascript project i didn’t understand why they use "slides.length-1".Why cant we give only slide.length only.why -1 should be included in the code can anybody explain?

const next = document.querySelector('.next');
const prev = document.querySelector('.prev');
const slides = document.querySelectorAll('.slide');

let index = 0;
display(index);
function display (index) {
    slides.forEach((slide) => {
        slide.style.display = 'none';
    });
    slides[index].style.display = 'flex';
}

function nextSlide () {
    index++;
    if (index > slides.length - 1) {
        index = 0;
    }
    display(index);
}
function prevSlide () {
    index--;
    if (index < 0) {
        index = slides.length - 1;
    }
    display(index);
}

next.addEventListener('click', nextSlide);
prev.addEventListener('click', prevSlide);

>Solution :

This is all because we count indexes from 0

Imagine having an array of length 5. This means that array has 5 elements.

But their indexes are: 0, 1, 2, 3, 4 (five elements in total).

That’s why when you want to access last element of any array you use:

myArray[myArray.length - 1]

which is literally myArray[5 - 1] so myArray[4]. And as I mentioned, indexes in array of length 5 are: 0, 1, 2, 3 and last but not least: 4