A Reminders is a way of making sure that something gets followed up on. You can create an unassociated reminder or you can associate a reminder to a person, organization or job.
If what you want to remember is associated with a particular job or person: If you right click on a job or open a job file you can select add a reminder. Chronicle then knows that reminder goes with that person or job. These reminders will be delivered at the time you select to your email tab in Chronicle where you can view the panel information for the specific entity you set the reminder for.
If what you want to remember isn't associated with a particular job or person: You can go to your tools list and select reminder to create an unassociated reminder. For example, if you wanted to remind someone to take the truck in for servicing, you might use a reminder. (Or you might use a task for this.)
If you want the message to regularly repeat: You can even set reminders to repeat and be delivered at intervals. To do this you need to have added a reminder field to your InfoPanel on the home page. Double click on the field and it will open the reminder tool for you so that you can do that.