Thou hast forgiven the iniquity of thy people, thou hast covered all their sin. Selah. Thou hast forgiven the iniquity - נשאת עון nasatha avon, Thou hast borne, or carried away, the iniquity. An allusion to the ceremony of the scapegoat.
Thou hast covered all their sin - As thou hast freely forgiven it, its offensiveness and abominable nature no longer appear. The whole is put out of sight; and, as we are restored from our captivity, the consequences no longer appear.
Selah - This is true. Our return to our own land is the full proof.