As Jae-Woo explained, it is used to explain you are in the middle of doing something. The form is ~(는) 중이다. It is similar to ~고 있다, but has a much stronger nuance because 중 has the literal meaning of center/middle. (인제) 영화 보는 중/중이야~ (I'm in the middle of watching a movie (now)) (인제) 영화 보고 있어~ (I'm watching a movie (now))

You are in the middle of an action.. Now I'm having an interview. Like in English they say progressive actions diversely 나는 집에 가는 중이야, or 가고 있어. I'm going back home now I'm on my way home now