It's one of attributive endings (
관형 어미) which transform a predicate (verbs, adjectives) into a modifier of the following noun. When attached to a sentence, they make the sentence something like a relative clause in English.
There are four common attributive endings
-는. They express different tense and aspect/modal.
-ㄴ/-은 implies a true statement, meaning that X DOES actually have the property of what the predicate states. Can roughly be mapped into present perfect in English - something happened in the past and still effective.
- 존이 집에 간 것을 알고 있어요?
Do you know the fact that John has gone back to home?
- 활짝 핀 꽃
fully bloomed flower
- 깨끗한 책상
a neat desk
-ㄹ/-을 attributes possibility, meaning that X may or will have the property. Adjectives do not go with
- 존이 집에 갈 것을 알고 있어요?
Do you know that John will go back to home?
- 그런 일이 일어나면 존이 집에 갈 것을 알고 있어요?
Are you aware that, in such a case, John may leave to home?
- 활짝 필 꽃봉우리
a bud that will bloom
-던 attributes some property that was true at some point in the past, but not likely at present. Can be roughly mapped into past perfect, or past progress.
- 여기가 수잔이 살던 집이다.
This is the place where Susan used to live. (she doesn't any more.)
- 깨끗하던 책상
a desk that were neat
-는. It's the attributive form from the present tense sentential ending (
-ㄴ다/-는다. So it can express
- something happens now
- something is determined to happen soon
- something happens habitually
You can think of present tense in English, and they will mostly fit. And again, adjectives do not go with this.