jQuery easing function — variables' comprehension

Question

How does the easing function for jQuery work? Take for example:

easeInQuad = function (x, t, b, c, d) {
    return c*(t/=d)*t + b;
};

How does that work? What does each parameter hold? How would I implement some dumb easing for an animation?

Also how would I attach an easing pattern to jQuery, is loading it into $.easing good enough?