People talk about pi a lot, and you might know pi is approximately 3.14 without being really sure why we care about it. Pi, also written with the Greek letter π, is the ratio of the circumference to the diameter for any circle. What that means is that for any circle, if you measure its longest distance across (its diameter), there are pi (about 3.14) of those lengths going around the circle (its circumference). Here's a visualization by John Reid & Arpad Horvath:
Here are various cool videos I found on the web about pi. I hope you enjoy them. Let me know of any others you especially like!
Richard Bew does a quick explanation and history of pi in his video Pi explained in 3:14 (YouTube video no longer available to embed; link goes to a commercial site).