3D coordinate system
Just like working in 2D, working in 3D means using a certain axis system. Unlike 2D, the coordinate system doesn't originate at the top left corner. But because we will be using a camera to view our scene, the origin doesn't matter that much.
If we hold the phone in landscape mode, the x axis is on the horizontal plane, the y axis on the vertical plane, and the z axis goes through the screen. This is known as a right-handed axis system.