The density is an unique property of a compound in solid or liquid phase. The density is the mass of the compound in its unit volume. Now due to the addition of an particular amount of solid like in this case salt (NaCl) the density of the water will change and the density given will be the density of the salted water.
The volume of the salted water is 1L or 1000mL. The density of the salted water is 1.058g/mL. Thus the 1 mL of the solution have the mass 1.058 g. Thus 1000 mL of the solution will have mass 1058.
