The Schuster-Schwarzschild model is a simplified model of the Sun's mechanism for producing absorption lines, treating the Sun as if a distinct outer layer (termed a reversing layer) produces the lines through opacity at specific wavelengths, which surrounds a black body producing a continuum emission. It was inspired by Kirchhoff's laws. The method has been refined and can still be used, e.g., for calculating a curve of growth. It can be considered an example of a two-stream approximation.
It is a simplification because real stars do not have such neat distinction between the source of the continuum emission and of the spectral lines. A more complex model of line formation that does not make this artificial distinction is the Milne-Eddington model. These were early-twentieth century models and today's computation capabilities make more complex and realistic models practical.
The original model simplifies by ignoring details of a ray's travel through the reversing layer such as the ray's angle and the thickness of the layer, effectively making it an early two stream approximation, and sometimes the term Schuster-Schwarzschild model is used for that aspect of the model even in cases where modeling absorption-line formation is not the goal. This is common in some (non-astrophysics) branches of applied science.