The adverb en masse means all together, like when the priest asked if anyone objected to your wedding during the ceremony, and the audience raised their hands en masse.
En masse comes from French literature, from the phrase "in mass." In this case, mass means "all the people." So, when you do something en masse, everyone does it together. Voting en masse means an entire group of people votes the same way. If everyone stands up and leaves a place en masse, it might be hard to get out the door.