It means because/due to/since/ something .
I can't answer your phone because I were driving my car
운전하느라/운전하는 중이였기 때문에 네 전화를 못받았다.
I stayed up all night because I played a game.
게임하느라 밤새 깨어있었다.
B is the result of A.
You can add verbs just in front of A.
먹느라 = 먹다 + 느라 because of eating
자느라 = 자다 + 느라 because of sleeping
하느라 = 하다 + 느라 because of doing something