Indeed, the reminder benefits the believers

(Whoever stands on the Night of Decree) meaning whoever revives this blessed night with prayer and recitation of the Quran.

"In faith and seeking reward", meaning: believing in the virtue of this night and the merit of deeds done in it, and seeking the face of God in His worship.

(He was forgiven for what had preceded of his sin) meaning: (God forgave him his previous sins)
The rights of people related to their money, honor, or bodies are inviolable; they cannot be waived except by their consent; thus, a person must seek forgiveness from those to whom he owes a right or fulfill the rights to their owners.