In that first link, where it says (referring to the diagram at the bottom right), "Observer moving parallel relative to setup [...]," I don't understand. Parall

In that first link, where it says (referring to the diagram at the bottom right), "Observer moving parallel relative to setup [...]," I don't understand. Parallel to what? Parallel to the mirrors? Parallel to the beam of light? I don't find it to be clear. Can you tell me which it is? Thanks. ------ P.S. Never mind. I follow the diagram now. I'm trying to follow the rest of it.