The mathematical pi is approximately the number 3.14159. The number itself is used as the ratio of a circle's circumference to its diameter. The number is actually much larger and is never ending.
A normal pie is a type of baked dish and has no relation to the number besides usually being a circle-ish shape.
Adding to the first response, the relation of the diameter of a circle (the line that passes directly through the circle to cut it perfectly in half) to the circumfrance (the line all the way around the circle), means that if you were to take the length of the diameter and lay it along the circumfrance as if it were a piece of string, you would have to use about three pieces of string that are the length of the diameter to stretch all the way around the circumfrance. So, pi is basically "the length of the diameter can go around the circumfrance 3.14159265... times." So, it's about three times.
