Pi is important in math because it describes how the circumference of a circle is in proportion to the diameter of a circle. To get pi, a mathematician divides the circumference of the circle by the diameter.
The shortened form of pi is 3.14 because the number is a irrational one. The number of decimal places in an irrational number continues on into infinity, so technically, pi has no end. It's also a number that shows up randomly in other places, equations and experiments, such as in Buffon's needle problem. Mathematicians have been familiar with pi for at least 4,000 years.