Working in passes is the controlled process of advancing your work one step (‘pass‘) at a time towards your vision. Every pass must be short, simple, and focus on improving just a few important aspects of the work.
To get a sense of what working in passes looks like, imagine seeing a cathedral from a great distance. You notice its general shape, size, height, but not much more.
Now imagine yourself walking towards the cathedral. Gradually, the features of the building become more and more defined; you begin to make out individual forms, spires, maybe some colors too. Then windows, doors, some of the more prominent ornaments. Now almost at the feet of the cathedral, you can appreciate the hundreds of fine decoration elements. Get even closer, and you detect textures and minor defects in the material itself.
An artist working in passes goes through a very similar experience as he creates his own ‘cathedral’.