It is a theorem in geometry that if you take any circle and divide the circumference of the circle by the diameter you always get this same number which we denote by the Greek letter "π" (pi).
π is known to be an irrational number. That means that the decimal expansion never ends and has no repeating pattern. It also means that it cannot be written as a fraction where both the numerator and denominator are integers (whole numbers).Therefore, you can't write out in numbers the exact value of π . There are ways to approximate it, however, to any (finite) number of decimal places you want.
Archimedes proved that π is little less than 22/7.