Moving Mountains
Moving a mountain is obviously a very big thing to do, so to say, "I'd move mountains for you" is the same as saying, "I'd do anything for you".
That's the sense in which Jesus used the phrase, when he told his followers about the power of faith. Even a tiny amount of faith can achieve huge things, he said...
I tell you the truth, if you have faith as small as a mustard seed, you can say to this mountain, "Move from here to there" and it will move. Nothing will be impossible for you.
Matthew 17:20