The scale of a map is the relationship between the distance on the map and the actual distance on the ground. It is usually expressed as a ratio or as a statement. In this case, the scale is expressed in statement form.
The statement "One centimeter on the map represents half a kilometer on the ground" means that the map has a scale of 1:50,000. This means that one unit of measurement on the map (in this case, one centimeter) represents 50,000 units of the same measurement on the ground (in this case, half a kilometer or 500 meters).
In simpler terms, if you measure a distance of one centimeter on the map, it represents a distance of 500 meters (or 0.5 kilometers) on the ground. This scale is commonly used for topographic maps and other detailed maps that show a smaller area.