### What does r in an answer mean

After I ran the following I got a valid image but what do the r's stand for in the solve part? They don't seem to have a value but I didn't get an error.

var('x y')
show(plot3d(x^2 + y^2, (x,-2,2), (y,-2,2)))

[image]

solve(x^2 + ~~y^2==0,[x,y])
~~y^2==0,[x,y])

[[x == r1, y == I*r1], [x == r2, y == -I*r2]]