The feature in WhatsApp Beta for Android version 18.104.22.168 allows users to be able to log in to the same WhatsApp account on two devices at once. The way to connect is similar to when trying to connect a WhatsApp account to the web version with Link Devices.
So, WhatsApp users who want to use their accounts on different smartphone devices don't have to bother having to move their main data and the like again.
This Companion Mode feature is said to be able to integrate up to a maximum of 4 different devices. In addition, the user's private messages and calls are still end-to-end encrypted even if they use WhatsApp from a spare smartphone.
Because it's only Beta, so only a number of WhatsApp users have just gotten this feature. In the meantime, just wait until the feature finally arrives for all WhatsApp users in the future.