It's a grammar point. Following verbs, ~는지 알다/모르다 is used to say you know how to do something, why something happens, or other knowledge-based things. Depending on the verb, this can be shortened to just ㄴ지 알다/모르다.
온돌을 설치하는지 알아요? - Do you know how to install the in-floor heating?
무슨 말인지 모르겠어요. - I don't know what you're saying.