That, my friend, is one beautiful question and I will try to answer it as best as I can in as few words as possible but as many as needed for you to 'truly' (your words) get the idea.
Our universe is a funny place. Apparently there are some things which just are the way they are and nothing we could ever do will change that. Mathematics is basically the motherload of unchangable consistency in the universe. Everything you'll ever do will be limited by the boundries of the material representation of mathematical systems known as physics.
Pi is an extremely elegant and easy to understand example of that. Pi, of course, has something to do with circles. Now, circles don't exactly exist in the 'real' world and what i mean by that is that there isn't a physical representation of a one-dimensional point or a one-dimensional distance on a two-dimensional plane by which the circle is defined. So really when we say circle in mathematics what we mean by that is a two-dimensional structure which consists of a center represented by any one-dimensional point you can imagine and all the points that have a certain fixed distance from that center-point. What's funny about all that is that you can arbitrarily choose any point you want and any distance from that center-point to 'build' a circle and even tho you can do this as often as you want and with as many different values and positions there's one property which you have no influence over and that, my friend, is the ratio between the diameter (or twice the radius) and the circumference of a circle, which is what pi is. Pi is an unchangable ratio a so called 'constant'.
