A 3D coordinate system describes positions using an origin and three axes, usually called X, Y and Z.
Three numbers need a frame of reference
Imagine placing a small sculpture beside a painting. You need to describe its horizontal offset, height and depth. A coordinate system gives those directions a consistent meaning. The position (0, 0, 0) is the origin; other positions are measured from it. The letters alone do not tell you which direction is up: Blender commonly uses Z-up, while glTF uses Y-up.
Why an imported object can arrive on its side
A model can look correct in its authoring application and face the wrong way after export if axis conventions are handled incorrectly. Moving the camera will not repair the model’s orientation. Check the export convention, the object’s rotation and its parent transform before adjusting individual parts. Axis colours are useful visual hints, but the labels and coordinate convention are what matter.
An art example
For a Painta composition, first decide which direction should come out from the artwork and which should follow its height. Then position the digital elements consistently in that reference frame. A small test object makes this easier to judge than a complex animated model. The diagram uses a labelled Y-up convention; it is an explanation of coordinates, not a screenshot of editor controls.