Radius, diameter, PI, and ² are all mathematics and knowledge that should be learned in school. These are not programming terms. I'll try to explain what I can about them.
Diameter is the distance from one edge of a circle to the direct opposite edge of the circle, so that the line goes directly through the center point of the circle.
Radius is half the diameter, or the distance from the center point of a circle to its edge.
² is an exponent, meaning multiply the number by itself that many times. Where 5² = 5 * 5 = 25 or 5³ = 5 * 5 * 5 = 125
So in the case of finding the area of a circle you would multiply the Radius by itself, so, where r would represent the radius, r² = r * r
I'm not going to attempt to explain PI. Just know that it is a number that represents the constant ratio to be multiplied by to get the area. It is often rounded to 3.14
I suggest that you review the course to see how to get the input, do the conversion, and output the calculation.