It can mean absolutely nothing, or just another place on the scale. Here it means the bottom of the scale. Zero represents whoever has the lowest standard of living in your town. Or in your country. Or in the world.
Think of Zero as the bottom rung of the ladder. We’re all somewhere on that ladder so we don’t want to break it, we’re just looking for some improvements. Specifically for that bottom rung. Lifting that bottom rung doesn’t mean removing all the other rungs, just the opposite. It requires those other rungs and it lifts all those other rungs when it rises.