A two-stream approximation is a type of approximation of electromagnetic radiation intensity as a function of direction, i.e., a description of how intense the light is when viewed in different directions, such as within an atmosphere. Physical reality can be subtle and complicated and a two-stream approximation is a simplification that makes calculations of radiative transfer and absorption lines tractable. It is "two-stream" for making a distinction between "down" and "up", essentially, assuming there is a portion of the light that is moving up, or away from the center of the object. I gather this simulates the effect of diffusion of photons in their random walk and/or the effect of the temperature gradient, both being overall effects that models focused on local phenomena would miss. There are variants, two of which are the Eddington approximation and the Schuster-Schwarzschild model.