하던 is like "something (I, You, They) were doing" or "used to do". 던 attaches to a verb stem to create a past continuous tense as an attributive adjective (i.e. modifying a noun), to indicate that something was going on for a while but not anymore. It is one of the important tense forms and the exact meaning can change with the verb form, as in 하던, 했던, 했었던. e.g. - 하던 일 다 끝내고 갈게 = I'll finish what I was doing and leave. - 어릴 때 같이 놀던 친구들 = Buddies I used to play with as a child. - 고등학교 시절에 다니던 교회 = a church I used to go to when I was in high school.