Pi is a number that is approximately equal to 3.14. It is the number you get if you divide the circumference of any circle by its diameter. It's the same for all circles. You can approximate pi for yourself by taking some circular things like the tops of jars and CD's and frisbees and measuring their diameter and their circumference. When you divide the circumference by the diameter, you'll get an answer something like 3.14. It will be the same every time (unless you measured wrong).
Actually, 3.14 is only approximately equal to pi. That's because pi is an irrational number. That means that when you write pi as a decimal it goes on forever, never ending and never repeating itself. The first six digits of pi are 3.14159, and that's all you need for most practical purposes. In most cases just 3.14 is enough.
Check out this webpage with a million digits of pi.
Usually in math we write pi with the Greek letter π, which is the letter "p" in Greek. You pronounce it "pie", like apple pie. It is called pi because π is the first letter of the Greek word "perimetros" or perimeter. But it was not the Ancient Greeks who first discussed the value of pi. Mathematicians in the Babylonian Empire, about 2000 BC, had already figured out that pi was about 25/8, or 3.125. By about 1700 BC, in the Middle Kingdom, Egyptian mathematicians calculated pi to be about 3.16. People have been gradually getting closer ever since, with early contributions from mathematicians working in China, India, and the Islamic Empire. By 263 AD, the Chinese mathematician Liu Hui had calculated that pi was 3.141.
To find out more about pi, check out these books from Amazon.com or from your library: