After completing Inside Job and making enemies with The Institute (see Banished from the Institute), this quest will start. Talk to Preston Garvey to update the quest. You have to recruit more settlements for the minutemen. Just do the requested settlement quests until you have at least 8 settlements under your control.
If you have already 8 settlements, this quest should be skipped; otherwise, if you continued through the other factions' quests and you still have this quest triggered, you have to do some settlement quests until this quest is updated. Report to Preston and he'll tell you about The Institute's planned major attack on The Castle. This will complete the quest and start the next one.