If you measure around any circle (circumference), measure across the same circle (diameter), and divide the first measurement by the second, you always get the same number, about 3.14. It doesn’t matter how big or small the circle is, the ratio of circumference to diameter is always about 3.14.
Well, mathematicians love constants like that, and somewhere along the line, they gave that number its own name, π (pi) a Greek letter. Some people have theorized that it was named π for the first letter in peri, meaning around (like perimeter).
On to Radians