Starting today, WhatsApp is adding end-to-end encryption to your cloud backups on both Android and iOS. While WhatsApp itself has been end-to-end encrypted by default for the past five years, until now if you chose to backup your chats to Google Drive (from the Android version of WhatsApp) or iCloud (if you're on iOS), those backups wouldn't be encrypted.
Now, however, you can turn on end-to-end encryption for your backups. The way WhatsApp owner Facebook describes this in its announcement, it seems like it won't be on by default but you'll have to actively choose to enable it by going to Settings > Chats > Chat Backup > End-to-end Encrypted Backup.
When you do that, you'll need to use either a password of your choice or a 64-digit encryption key that only you know. Once you set all that up, neither WhatsApp nor Google or Apple will be able to read your backups without your key or password.
Facebook rightly points out that WhatsApp is the only global messaging service to provide such a level of security at such a scale. The new feature allowing for end-to-end encryption of backups will be rolling out slowly to those with the latest version of WhatsApp for Android or iOS.