This is simply a way of tagging a team member in your candidate notes. Type your message and then type the ‘@’ symbol followed by their name. A dropdown should appear of all hiring managers or admins that have been added. Now the team member gets an instant notification with your notes about that wonderful new candidate.
To @mention a team member, they do need to be ‘Admins’ or ‘Hiring managers’ of the job the candidate has applied to.