Psalm 63:5 refers to the richest part or the fatness of the Soul. In the physical body, the marrow is the strength and vitality of the bone. Marrow helps produce blood cells, and stores fat; the fat is used as an energy source when needed.

The Word of God when it is fed to the Soul, works in the same way as the marrow in our bones. You can get fat in your Soul when you are properly fed by the Word of God!