First, basic geometry applies to an understanding of space. One of the features of space is if it's a plane. Think of the difference between a dot and a line. What's the difference? A line has one more dimension. Add a dimension to a line, and you get a plane.
Add a dimension to a plane, and you get what is known as a three-dimensional object. You can be one of those. Think of it, you have the side-to-side movement, the forward-backward movement, and you can even jump up and down. Everything else will be a combination of those.
Except for the Astral. Imagine adding another dimension to life, like making a plane out of a line. You can look up a Tesseract if you still want to stick to the geometry metaphor. If you find a representation of a Tesseract, imagine the small cube as real life and the Astral Plane as the bigger cube.