Pi is the name for the Greek letter π, which corresponds to the English letter p.
π is also the symbol used in mathematics for the ratio between the diameter of a circle and its circumference, and which appears in many other places.
π is an irrational number; this means that it cannot be expressed as a fraction, and (therefore) cannot be expressed exactly as a decimal no matter how many decimal places it is carried out to.
The value of π is approximately 3.14159. This value is precise enough for almost all ordinary purposes; it can, for example, be used to calculate the circumference of the Earth with an error of only 350 feet.
For rough purposes, the fraction 22/7 (= 3.14285...) is sometimes used.
In the Bible, 1 Kings 7:23 contains the famous passage "And he made a molten sea, ten cubits from the one brim to the other: it was round all about, and his height was five cubits: and a line of thirty cubits did compass it round about." If it assumed that "round" means "circular," it seems to imply that the value of π is exactly 3.