This is going to be the easiest technique project in the entire series, and I expect you're extremely relieved to hear that because it can be hard going always having to learn new things!
What you're going to learn about are local notifications, which let you send reminders to your user's lock screen to show them information when your app isn't running. If you set a reminder in your calendar, making it pop up on your lock screen at the right time is a local notification.
These aren't the same as push notifications, and in fact they are quite a different beast from a development perspective. I would love to cover push notifications here, but they require a dedicated server (or service, if you outsource) to send from and that's outside the remit of this course. Much later on – project 33 to be precise – we look at CloudKit, which can send push notifications when data is changed, but I wouldn’t recommend skipping ahead.
To get started, create a new Single View App project in Xcode, and name it Project21.