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]]
"Blotted out" is a metaphor, as if our sins were recorded with ink, but when God forgives our sins, they are removed, or erased. This can be stated in active form. AT: "so that God may remove your sins" (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"