"Pi" is a name for a number. The same way "a dozen" is twelve, or "a pair" is 2, "pi" is 3.14159... and the decimal keeps on going infinitely--there is no end, and no repeated pattern in the digits. (This is why it's called an irrational number. Because it is an infinitely continuing decimal, you can't write it as a simple ratio (fraction) because its exact value can't be calculated...)
Pi is the main puzzle piece in figuring out certain measurements of circles:
The circumference of a circle (its perimeter) will ALWAYS be its diameter x Pi, and will be represented as ¶d. (That's not a good Pi sign, but the real one doesn't come out well here!)
For example a circle with a diameter of 6 inches will have a circumference of Pi x 6, which multiplies out to 18.849... square inches.
As your circle gets bigger or smaller, so does its diameter and its perimeter, but the relationship between them (Pi) stays the same.
To figure out a circle's Area, the formula is "Pi, times the radius squared," or ¶r2. That means a circle with a DIAMETER of 6, which means its RADIUS is 3, would have an area of Pi x 32, or Pi x 9, which multiplies out to 28.274....
Again, as the circle gets bigger, so does its radius, and therefore its area, but the relationship to Pi stays the same.
Anyway, if you're more of a numbers person, forget you ever heard the word Pi and just swap in "3.14" and you'll be fine.
Matt in New York