Meridians are energy pathways that run throughout the body. In shiatsu what we're doing is balancing the two flows of energy, yin and yang, to find that balance.
Coming up the front of the body are the yin meridians. There are six of them running from the inside of the leg, front of the leg, all the way up to the head and also through the inside of the arms. On the back of the body the yang meridians run over the head and down the backs of the legs into the floor.
So when working with meridians we want to follow this natural course of energy. For example, if I wanted to work on the lung meridian, which starts right here in the chest and moves out through the inside of the arm, I would use various techniques such as pressure points, or I could do a nice stretch, but always moving in that natural flow with the meridian. And vice versa, if I wanted to work on the back of the body, instead of starting down and moving up, to go with the natural flow I would start my pressure points up and move down towards the floor.
So, that's the most basic way to work with the meridians to find ultimate balance in the body. Another thing about meridians, we can think of if there were rivers running throughout her body and she felt pain in a certain area, say like the top of her shoulder. In Western body work forms we might say there's a knot in the muscle. In shiatsu we would look at that as a dam or a block within a river that's stopping the flow of water. So if this was the dam in the river or the block of energy, in shiatsu by applying pressure there it helps free it up and redistribute the energy throughout the body.
So that's the basic principle of working with meridians in shiatsu.