"The letter I have sent."
Translation:내가 보내준 편지.
Im not exactly sure how to explain it but it means something like "to have done something/for someone" 주다 is a verb meaning "to give". So in special cases it is applied to verbs to imply it was done and given, in this case to send. Another example of this is with the verb "to help" 돕다. After conjugating it with "give" or 주다, it becomes 도와주다, or "please help". Honorific 시 is often added so 도와주시다 or 도와주세요! I hope this helps and isnt just confusing...