What are level curves of $f(x,y)=\frac{x^3-x^2y+xy^2-y^3}{x-y}$? 
Find the level curves $f(x,y)=k$ for $k=1,4,9$

My problem is that I don't understand what they mean by level curves and therefore I can't answer this question. Can someone please explain what they mean by level curves here?

Since:
$$
f(x,y)=\frac{x^3-x^2y+xy^2-y^3}{x-y}=\frac{(x-y)(x^2+y^2)}{(x-y)}=\mathbf{1}(x\neq y)(x^2+y^2)
$$
So they are circles without points on the line $x=y$.
