It basically means to be calm and focused, and to have yourself present and not day dreaming or wearing yourself too thin by feeling worried or stressed out. When you're in a bad mental or emotional state, it's hard to do an objective reading. Same thing goes for if your mind is floating elsewhere, and you have to come back down to earth, so to speak.
I guess it's the element of Earth, to feel your feet are firmly planted on the ground, and that you're balanced (centered), and stable, like a strong four walled house. It's common for people to meditate to ground themselves, which is basically preparing themselves first so they can feel ready and to conduct the reading afterwards.
I think it's optional to do this. There are many ways to calm yourself, like listening to music, taking a walk in the park, smelling scents of some kind, looking at images, etc., or you can just go right into the reading. That's up to you.