What is the meaning of 3d graph?

edit

I read the reference manual. There I see only x,y. So how is the value of z decided? Example:

def f(x,y):
      return math.sin(y*y+x*x)/math.sqrt(x*x+y*y+.0001)

P = plot3d(f,(-3,3),(-3,3), adaptive=True, color=rainbow(60, 'rgbtuple'), max_bend=.1, max_depth=15)
P.show()

I also don't know how to paste the sage cell code, as only latex is the way here.