
Explanation:

Box 1: CSV
Add multiple users in the Microsoft 365 admin center
* Sign in to Microsoft 365 with your work or school account.
* In the admin center, choose Users > Active users.
* Select Add multiple users.
* On the Import multiple users panel, you can optionally download a sample CSV file with or without sample data filled in.
* Etc.
Note: More information about how to add users to Microsoft 365
Not sure what CSV format is?
A CSV file is a file with comma separated values. You can create or edit a file like this with any text editor or spreadsheet program, such as Excel.
Box 2: User Name and Display Name
What if I don't have all the information required for each user? The user name and display name are required, and you cannot add a new user without this information. If you don't have some of the other information, such as the fax, you can use a space plus a comma to indicate that the field should remain blank.
Reference:
https://learn.microsoft.com/en-us/microsoft-365/enterprise/add-several-users-at-the-same-time