The ContourPlot3D function plots level surfaces for a function of 3 variables if given just an expression to plot, rather than an equation:

ContourPlot3D[ x^2+y^2+z^2, {x,-1,1}, {y,-1,1}, {z,-1,1}]

Use PlotLegends -> Automatic to see a legend identifying the function values that go with each color:

ContourPlot3D[(x^2)+(y^2)+(z^2),{x,-1,1},{y,-1,1},{z,-1,1}, PlotLegends->Automatic]

Set your own values for the levels via the Contours option. Also notice how restricting the range of x gives a “cut-away” view of the level surfaces:

ContourPlot3D[(x^2)+(y^2)+(z^2),{x,0,1},{y,-1,1},{z,-1,1}, Contours->{.25,.5,.75}]

The ContourPlot function is the 2-variable equivalent of ContourPlot3D. Here are level curves for the tunnel-like auditorium function, confirming that you did pick them right  yesterday:

ContourPlot[ Sqrt[400-4*y^2], {x,-20,20}, {y,-10,10}]