WhatsApp added the ‘Pin Chat’ feature in its last update. This feature let you pin any conversation on top of the chat list when you open WhatsApp. Usually, the chat list is ordered based on the latest message received in the conversation. This way your conversation goes down in the list if any other conversation receives the message later to your last chat.
Pinning a chat skips this ordering and always stay ahead of the list. Unfortunately, you can only pin 3 conversations for now (It would be great if the pinning limit increased to 5.). But you always can ‘unpin’ the conversation and assign it to another. Anyway pinning many chats would be pointless. Scrolling across the numerous chats while chatting with few contacts frequently has now ended here with this pinning feature.
I got this feature after updating WhatsApp last night in Playstore. So if you don’t see the ‘pin’ button after selecting any conversation, try updating WhatsApp to the latest version from Google Play Store.