True 3D or stereo viewing assumes that we have two eyes which are at some distance of each other. One eye sees the picture from a slightly different angle than the other one. The brain finally fuses the two separate pictures into the final picture we perceive.
If you look at an object in the distance, like a mountain, your eyes are parallel, the object is too far away. If the object is near, say a pencil 20 cm or 8 inches from your eyes, your eyes turn inwards towards the nose, more so the nearer the pencil comes to the eyes. The “toe-in” angle is the angle the eyes depart from looking straight on and parallel and turn inward.
The brain uses this angle (apart from other cues) to create a stereo image. For us humans, this is all fixed at the distance between the eyes, which is about 6.5 cm or about 2-3/4 inches or so. We can create pictures that are taken at a greater distance than the eye distance. We get a 3D effect still farther away but we perceive it as miniature - we are giants.
We can also cheat a bit by making the “inter-eye” distance larger in relation to the distance of the object and move the camera angle inwards in an unnatural way - toe-in - and give the brain a bit to ponder how to interpret what it sees.
I hope this helps for a start.