What are the "models" and "residuals"?

The Legacy Surveys data reduction approach is to try to fit simple elliptical models to the galaxies. We adjust the shapes and colors of the models until they best match the observed images. The residuals are then image minus model. The models are simple enough that they don’t work in enough detail to work for large galaxies or spirals, but the galaxies we’re really interested in for study with DESI are tiny distant galaxies where the simple models are basically fine.