I have made comments on almost every line of code, explaining what each line does.

// Set slide # to 1
var slideIndex = 1;
// Show the current slide
showSlides(slideIndex);

// Next/previous controls
function plusSlides(n) {
	// Increase the slide index by 1 and show that slide
	showSlides(slideIndex += n);
}

// Thumbnail image controls
function currentSlide(n) {
	// Show the current slide
	showSlides(slideIndex = n);
}

// Toggle correct slides
function showSlides(n) {
	var i;

	// All of the slides in the document
	var slides = document.getElementsByClassName("mySlides");

	// All of the slide dots in the document
	var dots = document.getElementsByClassName("demo");

	// All of the slide captions in the document
	var captionText = document.getElementById("caption");

	// If slide # is greater than the total number of slides
	if (n > slides.length) {
		slideIndex = 1
	}

	// If slide # is less than total number of slides
	if (n < 1) {
		slideIndex = slides.length
	}

	// Hide each of the slides
	for (i = 0; i < slides.length; i++) {
		slides[i].style.display = "none";
	}

	// Replace each of the dots with the class "active" with ""
	for (i = 0; i < dots.length; i++) {
		dots[i].className = dots[i].className.replace(" active", "");
	}

	// Set the current slide to display as a block element
	slides[slideIndex - 1].style.display = "block";

	// Set the current slide's respective dot to the "active" class
	dots[slideIndex - 1].className += " active";

	// Set the caption text to the current slide's caption text
	captionText.innerHTML = dots[slideIndex - 1].alt;
}

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