You can pin a chat to the top of your chats list so that you can find them easily.
If you have pinned more than one chat, the chats will be in a descending order of when you pinned them.

To pin a chat
iOS
Swipe the chat to the right > tap “Pin”
Android
Tap and hold the chat > tap “Pin” icon on top right corner

To unpin a chat
iOS
Swipe the pinned chat to the right > tap “Unpin”
Android
Tap and hold the pinned chat > tap “Unpin” icon on top right corner