iCloud is a free service from Apple that allows you to share items between devices. It also enables users to back up settings and apps.
When setting up a device, you’ll be asked if you want to use iCloud. If you decline, it can still be enabled later by opening Settings, tapping your name at the top to view your Apple ID details, and pressing iCloud.
From here you can also choose what to share across your devices through iCloud, such as contacts, calendars, Safari data, and notes, to name a few.
For example, if the Reminders option is on, whenever an event is created in the Reminders app on your iPhone, it will also appear in the Reminders app on your registered iPad, iPod touch, or Mac devices. It will also sync with iCloud itself, meaning that if you have to restore your phone, the information will be restored too.