What is r and r2?

Answer: Since this is listed in geometry I'm assuming you are referring to a geometric shape.

Thus r is used to represent a radius.

r1 would be the initial or radius 1.
r2 would be the final radius or radius 2.

A radius is the distance from the center of a circle to its outer edge.

That is,

If you draw a circle and put a dot in the center
the radius is the distance from that dot to the edge of the circle.

The diameter would be the distance from one side of the circle to the other.
The diameter is always twice the radius.

D = 2r

Answer: In other contexts, "r" can stand for almost anything, but in geometry it is often used for the radius. If it is written as r2 (the 2 raised), then that means r squared, equal to r times r.