。
I'm really sorry.
Literal
Truly not-resolved-[polite].
すみません, literally 'it does not finish / is not resolved' (from the verb 済む, 'to finish, to be settled'), is the default polite 'sorry / excuse me.' The speaker implies their debt or infraction isn't yet resolved. Also used for 'thanks when inconvenienced' and for getting someone's attention. 本当に intensifies the sincerity.