"the light of his glory." God's glory is associated with a very bright light. The author is saying that the Son embodies that light and fully represents God's glory.
"glory, the image of God's being." The "the exact representation of his being" is similar in meaning to "the brightness of God's glory." The Son embodies the character and essence of God and fully represents everything that God is. Alternate translation: "glory and is just like God" or "glory, and what is true about God is true about the Son"
"his powerful word." Here "word" refers to a message or command. Alternate translation: "his powerful command" (See: [[rc://en/ta/man/translate/figs-metonymy]])
The abstract noun "cleansing" can be expressed as a verb: "making clean." Alternate translation: "After he had finished making us clean from sins" or "After he had finished purifying us from our sins" (See: [[rc://en/ta/man/translate/figs-abstractnouns]])
The author speaks of forgiving sins as if it were making a person clean. Alternate translation: "he had made it possible for God to forgive our sins" (See: [[rc://en/ta/man/translate/figs-metaphor]])