The box, covered with lead, contains the soil of Israel. The phrase coffer nephesh in Hebrew, refers to ransom. Literally the word coffer means ransom and the word nephesh means soul.
COFFER: Like all objects whose essential quality is that of containing, it sometimes acquires the symbolic character of a heart, the brain or the maternal womb. The heart, the first of these meanings, is a figure characteristic of the symbolism of Romanesque art.
In a broader sense, receptacles which can be closed up have, from the earliest times, represented all things that may hold secrets, such as the Ark of the Covenant of the Hebrews, or Pandora’s box.
(J. E. Cirlot, A Dictionary of Symbols, New York)