“Vider son sac”, literally “to empty one’s bag”, can be translated by “to get it off your chest”. It means “to say what’s heavy on one’s heart”. It makes people feel relieved, even it it is far from easy.
The origin of the idiom is interesting. In the past, lawyers would carry court documents rolled up in bags. At the court, they would take the rolls out as needed to read them in front of the judges or magistrates!