Remove the dross from the silver, and out comes material for the silversmith
Proverbs 25:4
Dross is, according to the Merriam-Webster's Dictionary, "the scum, or impurity, that forms on the surface of molten metal". This dross is useless to the metal. It makes it less valuable and, in some cases, makes it completely worthless. When the scum is gone, it makes the metal so much more valuable and usable.
This is much like it is in our own lives. Our lives are worthless and unvaluable because of all of the things that we have done to add dross to our lives. We add scum and impurities to our lives and make ourselves less important. When we allow God to come in and take out all of the impurities and scum in our life, we become more valuable. We are then able to be used by God for His purpose.
It's not a one-step process either. We have to be continually cleansed and made pure by God. We constantly need to be refined in order to keep our "usability" pure. When we allow God to change us into something better (which is all that he ever does), we allow ourselves to become more valuable, not only to others, but also to the kingdom of God.