This is a method for solving the Poisson-Boltzmann equation in a finite difference setting. Some of the earliest references to this method are from Gilson and Honig (Gilson MK and Honig BH, Calculation of electrostatic potentials in an enzyme active site. Nature, 1987. 330(6143): p. 84-6.). The method starts by solving the equation on a coarse grid (i.e., few grid points) with large dimensions (i.e., grid lengths). The solution on this coarse grid is then used to set the Dirichlet boundary condition values for a smaller problem domain -- and therefore a finer grid -- surrounding the region of interest. The finer grid spacing in the smaller problem domain often provides greater accuracy in the solution.
This technique has been used to cover multiple regions of a large problem domain in the parallel focusing method.