Although it is now an essential “iCloud” for the iPhone, its function is not yet known. There are many people who just see the words sometimes but don't know what to do just by following the guidance at the beginning of use.
iCloud is a cloud service operated by Apple (various programs executed on Apple's server on the Internet) and provides various functions to Apple products such as iPhone, iPad and Mac. If your iPhone is signed in to iCloud (authenticated with your Apple ID), it communicates with iCloud as needed to send and store various data.
Many of the apps that come with iOS are designed to work with iCloud. You can synchronize data with devices that can be connected to iCloud such as iPad and Mac by saving information such as address and name in “Contact” and event information in “Cloud”.
Conversely, if you are not connected to iCloud (signed out), you can only view the address you entered on your iPhone on your iPhone. Event information registered on the Mac will not be reflected on the iPhone. When you are signed in to iCloud, you can store data on the Internet and retrieve it at any time, but you will not be able to do so when you are signed out.
Note that even if you sign out of iCloud, you can no longer send or receive data, and the data will not disappear unless you delete the account (Apple ID). When you sign out, you will be asked if you want to keep a copy of the data on your iPhone, but the data will continue to be stored on iCloud. If you sign in to iCloud again, you can access your data as before signing out, so don't worry.