In Short: WhatsApp beta has just received a new update, v2.18.300, and that update brings the swipe-to-reply gesture to the app. This gesture has been available in an iOS version of WhatsApp for a long time now, and considering it has just arrived to WhatsApp beta, we can expect it to hit the stable version of the app in the near future. Various rumors have been saying that the feature will become available for an Android version of WhatsApp, and it's now happening.
Background: The name of this feature is kind of self-explanatory, but in case you're unclear on how it works, you should know that you'll be able to simply swipe / flick any message to the right, and once you do that, you'll be able to type your reply to that message. This will, basically, replace the tap-and-hold option that was available thus far, or tap the reply button in the top bar. Truth be said, both of those options will probably remain available to you, but this new version is quicker and more intuitive, at least WhatsApp thinks it is. In order to get the beta version of the app, you will have to sign up for the WhatsApp beta program via the Google Play Store (link available down below). If you're willing to wait a bit, though, the update is expected to arrive to the stable channel soon.
Impact: WhatsApp is currently one of the most popular messaging platforms on Android, though this app does much more than that, of course, you can take voice and video calls with it as well. WhatsApp is rumored to get ads in the near future, at least iOS version of the app, which probably means that Android is next in line, though hopefully those rumors are wrong. In any case, this new feature has been quite successful for WhatsApp on iOS, so the company is hoping that Android users will like it as well.