Once you have a check-in configured, everyone you've selected to fill it out will receive a reminder each day it's due.
By default, each person will receive a reminder by email:
And if you have Slack connected, each person will receive a DM on Slack (note that the Jell Slack bot doesn't support check-ins at the moment):
Each user can configure where they receive reminders by going to Notifications from the menu:
From here, they can toggle if they get notifications from email or Slack: