Adding reminders against users can be used for things like MOT's, birthdays, or even performance reviews. You can set this up through your Company settings, scroll down to 'Users' and open the profile of the user you'd like to add a reminder for. From their profile, click into the 'Reminders' tab.
You can set a date for the reminder, and a date for the user roles to receive a notification regarding the reminder, these details can be changed at any time. Once you've filled out the necessary fields (the required, mandatory fields are marked with an asterisk), click 'Save'.