having problems understanding a tutorial on one page scroll-vanilla JS- no libraries

Question

I was looking into one of the only few full page scroll codes which I have found so far, and wanted to somehow get the idea of what I should do by reversing the engineering but I'm noob and not good at understanding it pretty well, I don't understand it well and I get stock at every line, can you please explain to me what the guy has done? Whats the basics of what he is doing ..variables and functions and what they are in this case and why we need them? A summary on what is going on. this is the link to the code

https://codepen.io/igstudio/pen/pbYOab

this is the JS code

(function() {
    "use strict";
    /*[pan and well CSS scrolls]*/
    var pnls = document.querySelectorAll('.panel').length,
        scdir, hold = false;

function _scrollY(obj) {
    var slength, plength, pan, step = 100,
        vh = window.innerHeight / 100,
        vmin = Math.min(window.innerHeight, window.innerWidth) / 100;
    if ((this !== undefined && this.id === 'well') || (obj !== undefined && obj.id === 'well')) {
        pan = this || obj;
        plength = parseInt(pan.offsetHeight / vh);
    }
    if (pan === undefined) {
        return;
    }
    plength = plength || parseInt(pan.offsetHeight / vmin);
    slength = parseInt(pan.style.transform.replace('translateY(', ''));
    if (scdir === 'up' && Math.abs(slength) < (plength - plength / pnls)) {
        slength = slength - step;
    } else if (scdir === 'down' && slength < 0) {
        slength = slength + step;
    } else if (scdir === 'top') {
        slength = 0;
    }
    if (hold === false) {
        hold = true;
        pan.style.transform = 'translateY(' + slength + 'vh)';
        setTimeout(function() {
            hold = false;
        }, 1000);
    }
    console.log(scdir + ':' + slength + ':' + plength + ':' + (plength - plength / pnls));
}
/*[swipe detection on touchscreen devices]*/
function _swipe(obj) {
    var swdir,
        sX,
        sY,
        dX,
        dY,
        threshold = 100,
        /*[min distance traveled to be considered swipe]*/
        slack = 50,
        /*[max distance allowed at the same time in perpendicular direction]*/
        alT = 500,
        /*[max time allowed to travel that distance]*/
        elT, /*[elapsed time]*/
        stT; /*[start time]*/
    obj.addEventListener('touchstart', function(e) {
        var tchs = e.changedTouches[0];
        swdir = 'none';
        sX = tchs.pageX;
        sY = tchs.pageY;
        stT = new Date().getTime();
        //e.preventDefault();
    }, false);

    obj.addEventListener('touchmove', function(e) {
        e.preventDefault(); /*[prevent scrolling when inside DIV]*/
    }, false);

    obj.addEventListener('touchend', function(e) {
        var tchs = e.changedTouches[0];
        dX = tchs.pageX - sX;
        dY = tchs.pageY - sY;
        elT = new Date().getTime() - stT;
        if (elT <= alT) {
            if (Math.abs(dX) >= threshold && Math.abs(dY) <= slack) {
                swdir = (dX < 0) ? 'left' : 'right';
            } else if (Math.abs(dY) >= threshold && Math.abs(dX) <= slack) {
                swdir = (dY < 0) ? 'up' : 'down';
            }
            if (obj.id === 'well') {
                if (swdir === 'up') {
                    scdir = swdir;
                    _scrollY(obj);
                } else if (swdir === 'down' && obj.style.transform !== 'translateY(0)') {
                    scdir = swdir;
                    _scrollY(obj);

                }
                e.stopPropagation();
            }
        }
    }, false);
}
/*[assignments]*/
var well = document.getElementById('well');
well.style.transform = 'translateY(0)';
well.addEventListener('wheel', function(e) {
    if (e.deltaY < 0) {
        scdir = 'down';
    }
    if (e.deltaY > 0) {
        scdir = 'up';
    }
    e.stopPropagation();
});
well.addEventListener('wheel', _scrollY);
_swipe(well);
var tops = document.querySelectorAll('.top');
for (var i = 0; i < tops.length; i++) {
    tops[i].addEventListener('click', function() {
        scdir = 'top';
        _scrollY(well);
    });
}
})();

Thanks.