f=Abs[x y]

Plot3D[f,{x,-1,1},{y,-1,1}]

Plot3D[Piecewise[{{y,x>0∧y>0},{-y,x>0∧y<0},{y,x<0∧y<0}, {-y,x<0∧y>0}}],{x,-1,1},{y,-1,1}]