which values of c is the locus of the function (x^2+x+y+3)y+z(x+y+3)+c a surface??

I must have missed a lecture on this, because i have no idea what the question wants me to do. I looked up what a locus was... and I'm still not sure. I looked through my textbook and found nothing, so I'm really confused, can anyone help start me off, by telling me what I have to do? please