This is another idiomatic phrase that would probably make little sense to non-English speakers. A rolling stone would probably suggest momentum or personal drive, but the moss gathering part would be confusing, I'd think. Moss growing on a rock near a stream isn't such a bad thing, but if you look at the idiom symbolically, it suggests remaining sedentary and stagnant is not good.
I generally agree with the sentiment behind "a rolling stone gathers no moss", but I also think some people take it too literally. They never slow down, even if there are times when a little throttling back would be helpful. A rolling stone may not gather moss, but it also doesn't gather other important things, either.