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))