Okay, so what “seems” to happen is that the server has a cube that is not moving at the place where it is when you press Play and for the client it seems that the cube is moving.
The client side is thinking “Here’s a cube that’s moving, so I cannot walk past it” and it doesn’t let the player walk through it. Meanwhile the server is just chilling “Yeah, you do you”.
Next the client tries to walk to an “empty” space where he doesn’t see a cube (since it has moved on the client side) and the server goes “Oy! There’s a cube there, so no can do, boyyy!”.
Basically the server has a cube and says to the client “There’s a cube there, so you cannot enter that space” and the client side goes “I have a moving cube so I cannot walk past the moving cube”, but since the server decides what’s real and what’s a dream the server can walk on the space that the client currently has a cube.
Wow, that didn’t make any sense 
But yeah, those are still my thoughts, I think I get it though, even if my explanation is somewhat lacking. Hope nobody reads this tbh 