If a project has been set accept incoming email a user can add a note to a ticket by sending an email message to the ticket.
Where an email notification has been sent to a user about an update to a ticket, the user can reply to the email and the reply will be added to the ticket as a note. (see User Notifications)
It is also possible to copy the email address from the email to this ticket link icon ( envelope ) shown at the top right of the ticket page. (The email to this ticket link will only be shown if the project has been set to accept incoming email).