A roost is a place where birds or bats rest or sleep.

When birds or bats roost somewhere, they rest or sleep there.

If bad or wrong things that someone has done in the past have come home to roost, or if their chickens have come home to roost, they are now experiencing the unpleasant effects of these actions.

If you say that someone rules the roost in a particular place, you mean that they have control and authority over the people there.