Think back to your high school geometry class; Pi is the ratio of the circumference of a circle to its diameter. Pi is approximately 3.14159, but has been calculated to over two trillion digits, it goes on infinitely without repeating or becoming a pattern. Science would not exist without Pi. Mathematics, statistics, engineering, geography, geology, computer science, etc. all rely on Pi.
More information about Pi can be found at Wolfram MathWorld: http://mathworld.wolfram.com/Pi.html