The Chinese word for central equilibrium is zhong ding. It is a place and a concept. It is the center point of alignment. It is 3D, as Master George Xu says, meaning it can have volume, or more precisely, six directions: up/down, front/back, left/right simultaneously.
A basic concept of internal movement art is to move from a single point. The attention is placed and held on that point and the movement is initiated and the rest of the body is activated. Placing and holding the attention is a listening with your whole body, not just the ears. It’s like seeing with the ears and listening with the eyes.
To practice zhong ding, move just the spine. One vertebra at a time, like climbing a ladder. The attention climbs up and steps down each one at a time. A center line comes into your view upon which energy moves. It’s like a tube aligned along the spine from tailbone to top of head. This is the place of zhong ding. This line can go deep into the earth and high into the sky.
Progress to focusing attention on the central equilibrium of the legs. Ultimately you can shift your attention to anything, inside or outside the body, and give it a central equilibrium and move from there. This is the concept of zhong ding practice.
Just about any simple move will do. Perform slowly, softly, and intend on holding your gaze on the locus; in a beginner’s case, the spine area.