I have come to rely on the Reminders app on my iPhone. I set reminders daily and particularly enjoy location-based reminders that remind me, say, to put the towels in the dryer when I get home. What I don't enjoy is getting the same reminder on my iPhone, iPad and MacBook.
With iCloud, you can sync the Reminders app so that you are alerted to reminders from all of your devices. Because my iPhone is rarely out of arm's length, I don't want or need reminders duplicated across my Apple ecosystem. Thankfully, it's easy to turn off these duplicate reminders.
On a Mac, open System Preferences and select iCloud. From the list, uncheck the box for Reminders.
Similarly, on an iOS device, go to Settings > iCloud. Then tap the toggle switch to off for Reminders.
In related news, learn how to restore your iCloud files, contacts, calendars and reminders.