Reminder is a scheduled notification that can be sent to any available recipient single time or recurrently depending on date and time based events.

Go to Process builder. Open Step Settings (1) and expand "Reminders" (2).

Click on "add" links (3) to open popup "Step Reminder Edit"(4):

Reminder can be scheduled for some period of time relatively to specified date/time (event). Period (5) must be a numeric expression that should return a number of units that should be selected in proper picklist (6). Period type (7) picklist allows to select if period should be set before or after event.

Event is a some date/time value. User can select it's type (8) depending on his needs:

•   Process - event value (9) will be related to the current process instance:

• Field - event value should be selected from current process Data Source Object fields (click ">>" button to show field picker (10), select necessary field and click "Apply" button):

• Formula - value must be returned as a result of formula expression (click ">>" button to show formula picker (11), write a formula and click "Apply" button):

Check "Consider Business Hours" checkbox (12) to exclude non-working hours, weekends and holidays (if set in the user's organization) from date/time calculations.

By default reminder is sent one time. User can make it recurrent (auto-sent periodically). Check "Is Recurrent?" checkbox (13) to show proper form. Set repeat  period (14) and specify stop condition (15) if necessary (use merge fields picker at the top of popup):

Configure notification.

Reminder can be sent to:

• Step Assignee - is taken from the current step automatically.
• User, Group, Role, Profile - should be selected from proper lookups or picklists.
• Field - should be selected from the list of Data Source Object fields and should represent any assignable object (user, group, etc.).
• Variable - should be selected from the list of variables defined in the process and should contain ID of any assignable object (user, group, etc.).
• Email Address - should be entered manually.
• Slack - the same Slack Webhook as selected in "Slack Setting" field of Process Notification Settings.

If reminder recipient is set to anybody that can be recognized by system (Step Assignee, User, Group, Role, Profile, Field and Variable), it can be notified via Chatter or email.
If recipient is set to "Email Address", it can be notified via email only.
If recipient is set to "Slack", it can be notified only by pre-selected Slack Webhook.

To send notifications via Chatter, just select proper option in "Remind By" picklist (16) and write a message text in the proper field (17):

To use Canvas Application, check "Enable Canvas" (18) and populate Canvas properties in small form that will appear below (19).

To send notifications via email, select "Email" option in the picklist (16). Then select Email Format (20), populate "Subject" (21) and "From" (22) fields:

If you select "Plain Text" in Email Format picklist, your message will be sent as is.
If you select "HTML Text", your message will be sent as HTML. Enter HTML code in the "Message" field (24). Also, the "Preview" button (23) will appear - click it to see preview of your message in a popup browser window.
If you select "Email Template", your message will be sent based on pre-defined template that should be selected from the proper picklist (25):

To send notifications via Slack, just select proper option in "Remind To" picklist and enter message.

Save configured reminder.

All reminders will be shown in step properties. They can be deleted (26), added to library (27), or edited by clicking on proper row (28):