A new feature has been added to the whatsapp application.
This feature allows users to pin some chats to the top of their chatting list, this means these chats stays on top despite the recency of the chat .
How to Pin Chat?
Tap and hold on any chat and you’ll get a new pin icon in the action bar, click on it to pin.
For now, only 3 chats can be pin after which you will get a notification popup saying you can't pin more.
If you want to checkout this feature before it goes official,
Download the latest WhatsApp Messenger 2.17.313 beta August 25, 2017 HERE