The words "Repent" and "turn" refer to changing one's way of thinking, turning away from sin and turning to God. It is a way of saying the same thing for emphasis. (See: [[rc://en/ta/man/translate/figs-parallelism]])
Here "blotted out" is a metaphor for forgiving. Sins are spoken of as if they are written in a book and God erases them from the book when he forgives them. This can be stated in active form. AT: "so that God will forgive you for sinning against him" (See: [[rc://en/ta/man/translate/figs-activepassive]]) (See: [[rc://en/ta/man/translate/figs-metaphor]])
"times of relief from the presence of the Lord." Possible meanings are 1) "times when God will strengthen your spirits" or 2) "times when God will revive you"