All of he custom fields are optional and you can add them to issue screens any time you want. You can find more details on how to add a custom field in JIRA's own documentation. To easily filter custom fields provided by the add-on enter 'Reminder' to filter section of 'Select a Field Type' dialog as shown in the following screenshot.

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Create Reminder 

This field can be added to workflow transition screens and used to create reminders as a part of workflow transition. Details of this field can be used in 'Creating Issue Reminders on Workflow Transitions' section.

Reminder Date

This custom field shows date of the first active reminder you have permission to view for the issue. The first active reminder means the reminder that will be trigger before any other reminder. Value of this field may be different for each user due to private reminders.

Reminder Summary

This custom field shows summary of the first active reminder you have permission to view for the issue. The first active reminder means the reminder that will be trigger before any other reminder. Value of this field may be different for each user due to private reminders.