A more thorough explanation:

Definition: En masse (en mas) is a French term that means "in a mass" or "all together." It refers to a large group of people or things that are gathered or moving together at the same time.

• The protesters marched en masse to the city hall.
• The birds flew en masse across the sky.
• The students left the classroom en masse when the bell rang.

These examples illustrate how en masse is used to describe a large group of people or things moving or acting together. In the first example, the protesters are all marching together to the same destination. In the second example, the birds are flying together in a group. In the third example, the students are leaving the classroom all at once when the bell rings.