A sandbag (floodbag) is a sack made of hessian/burlap, polypropylene or other materials that is filled with sand or soil and used for such purposes as flood control, military fortification, shielding glass windows in war zones and ballast.
Advantages are that burlap and sand are inexpensive, and that the bags can be brought in empty and filled with local sand or soil.
Read more about Sandbag: Usage, Sandbags Made Using Big Bags, Other Uses, In Slang