When the entry is saved, the user you mention will receive a notification and an E-mail notifying that they are mentioned in an entry by someone. If the user clicks the notification or the link in the E-mail, it will navigate to the entry, allowing the user to confirm the data, comment, and communicate with teammates. Users will be notified multiple times if they are repeatedly mentioned.
It is also worth mentioning that if a user does not have access rights to a sheet, you can still mention them in the entry, but they would not receive the system notification and the notification E-mail. Therefore, please make sure that the user has access rights to the sheet before using @mention.