Those of us who use WhatsApp a lot know that the conversation list can get pretty crowded very fast with groups, plenty of family and friends, and random individual chats. That leads the important chats, the ones you come back to many times during the day, to always change their order in the list and jump up and down depending on when the most recent message was sent. So you're left skimming the list with your eyes, and sometimes scrolling down, to find that conversation you need. (Or you add a chat shortcut to your homescreen.)
WhatsApp beta 2.17.162 (and now 163) solves that by adding one very handy little feature: pinned chats. Tap and hold on any chat and you get a new pin icon in the action bar next to the delete, mute, and archive functions. Once pinned, a conversation stays on top of the list regardless of the recency of the last message compared to other chats.
You can only pin 3 chats after which you get a small toast notification message saying you can't pin more. If you change your mind, you can always unpin one of your 3 chats and choose another.