I need help understanding a proverb.

I was born in America and my Vietnamese father died when I was a teen. I have asked my family and everyone just says my dad liked to kid around. My dad used to say, "The ox pulls the cart, and the montain is there." I asked him what it meant when I was young and he would tell me that I would understand when I grew older. Well, I am a over a decade older now, and I still not sure I get it. I have come up will all sorts of meanings over the years, but I just want to know what he meant, if anything.

Is this a real proverb? If so what does it mean? Was my dad just teasing us as kids?