The new WhatsApp feature was announced by Meta CEO Mark Zuckerberg. It has started rolling out, and the feature should reach everyone in the coming months. This feature lets you set a unique password for your locked chats. This passcode will be different from what you use to unlock your phone so you have another layer of privacy for your locked chats.
Once you use the secret code, you can set your locked chats to appear only when you type in the secret code. This is so that your locked chats folder is hidden from your chat list. You can still choose to have the locked chats folder appear in your chat list.
You can use the secret to open your locked chats. You can also hide your locked chats folder, and use the secret code to search for it. WhatsApp has also made it easier to lock chats. Now you just have to long press on a chat to lock it instead of opening the chat's settings.
Since the feature has just started rolling out it might take some time before it reaches everyone.