Good question. "Steady state" means that ocean saltiness is not changing --it is stable. At the same time that salt is entering the ocean (for example, from rivers) it is also being removed by other processes. Some elements, such as silicon and calcium, are used by marine organisms to build their shells or skeletons. Others, such as potassium and sodium, are absorbed onto small particles of clay and sediment and are deposited on the sea floor. The salt inputs and outputs balance each other, so that there is no net change in ocean saltiness.

In the 1700's geologists attempted to calculate the age of the earth by measuring the oceans' saltiness and volume, and estimating how long it would take for that much salt to be added. But they didn't realize that salt was also being removed, so their age estimates were much too young.