WhatsApp is growing its functionality and it finally got a swipe to reply feature. It allows you to use a swipe gesture to reply instead of a long tap on the message plus button tap.
Everything which helps you to deal with your day to day activity faster is a very good improvement. This feature becomes even more helpful in groups when you want to reply to the specific message.
In order to use this feature, you will need to have the latest beta app. If you are not a beta user you can check this post about "how to become a beta tester for WhatsApp".
This feature is a client-side and available to everyone. To use a swipe gesture to reply you need to swipe a message to the right. The app will also vibrate back and you will see a swiped message in the reply box. Normally I would do it before typing a message but if you typed your text and realized that you forgot to add a reference - you can always do it at any time.
I would also say that this feature is a must for a messenger these days and WhatsApp was a bit behind others. For example, Telegram X has this feature for a long time already.