ECN Forum Forums Technical Discussion Electrical Theory and Applications why do we parallel conductors?

First, I am not an electrician.

I;'m in HVAC.

My son-in-law bought a new mobile home. THE HOOKUP USE FOUR WIRES.I was told, two hots, one ground and one neutral. Now the neutral and the ground were put under the same lug. Both at the service loop and the distribution box.
HOW CAN THEY BE DEFFERENT, UNDER THE SAME LUG.
I always thought the were derrerent, but maybe not.
What is the derrerence between the ground and the neutral.