Here. If you take a piece of string that is 2 units in length (lets say meters) and make it into a circle (as close to a perfect circle as you can), then the radius of the circle will be 1/π because circumference is 2 * π * r. Basically, anything involving circles will have pi in there somewhere, because pi is a ratio, it is simply the circumference of a circle over the diameter of the circle. This works for any size circle.
Other things that involve pi are sin, cos, and tangent functions, which are used often in physics, where you are investigating something that has a cyclic nature or some kind of circular motion. (like a wheel turning, a sound wave, etc)