Selecting Mailbox size shows the size of each user's mailbox. You may want to leave this unselected if disk quotas are enabled, as the users' total disk usage will already be shown.

Selecting Last login shows the most recent login time for each mailbox user.

Selecting Databases shows how many databases each user can access, complementing the information already available in the Login access column.

Selecting Other plugins shows columns provided by other mailbox plugins, allowing extra per-user plugin access details to appear in the users table.