The cloud typically means cloud storage, or a way to store your data on an online server somewhere on the Internet.
The data on these online servers is usually encrypted to prevent others from accessing it.
It is not a good idea to use this as your only place of storing your data, as some viruses can corrupt these places, so it is always a good idea to keep regular backups.
Apple's iCloud
Apple makes use of the iCloud to store device backups, copies of the photos on devices and other settings including iCloud Drive, which can be used to store files and folders from a Mac to reduce the amount used up by them. Users can restore from an iCloud backup if they need to restore or replace a device.
Microsoft's OneDrive
Microsoft uses the OneDrive as the primary place to save files. In the event of a problem with the computer, the computer can be safely reinstalled or replaced and the files are accessed once the user logs back into their computer.
