은 is used like a relative pronoun in english. It is attached to an adjective and describes the following noun. Its form changes depending on the tense. 보고 싶은 사랑 means the love(maybe a person) whom I miss. Another example; 하고 싶은 말 the words which I like to say. 운동하기 좋은 날씨 weather which is good for exercise. 성격이 괜찮은 남자 a guy who has a good personality.

Giving ㄴ/은/는 makes the verb into an adjective. 보고 싶다 + 은 + 사람 = 보고 싶은 사람. It means a "person/someone" (사람) that you "want to see/miss" (보고싶다) Other example 예쁜 여자 = pretty woman (예쁘다 = to be pretty) 예쁜 여자예요. = It's a pretty woman. compare with 윤아 예뻐요. = Yoona is pretty 사랑하는 사람 = someone who love (사랑하다 = to love) It can also mean someone who is loved though, depending on the context To understand more, please learn about making adjectives from verbs grammar. Ask me if you have more question about it.^^