Sending reminders is a feature of Premium Doodle. and allows you to remind the participants who have not yet voted in your poll, to kindly do so. To be able to send reminders you have to send out your invitations via Doodle, rather than sharing the participation link. Click the 'invite' button at the top of the page and then 'email.' If you connect your address book, emails will autofill as you type. This is also the way you can track who is missing from you invitations. You'll see their email addresses listed as placeholders at the bottom of the table.
When you want to send a reminder to the participants who have not yet voted in your poll, simply open your poll, and click on "Send reminder" at the top of the page. (If you do not see this option, it might be because everyone have already voted, or you did not invite your participants through Doodle). You can use the default reminder message, or personalize it if you would like.