WhatsApp is one of the largest and most popular messaging services around. WhatsApp is owned by Facebook and is the only messenger on Android to actually hit a billion downloads. Additionally, WhatsApp is the only app from Facebook that is not blocked in China. That may not seem like a big deal, but when talking about the sheer size of the users for WhatsApp, it's pretty important. Since being acquired by WhatsApp, we've seen quite a few changes hit the service and the app itself. It got a bit of a redesign about a year ago. While it isn't material design, it's much better than what it used to be. We also got the ability to use WhatsApp on the desktop, and now we are getting a beta track in Google Play.
A few years ago, Google I/O 2014 to be exact, Google announced beta tracks in Google Play. Essentially what it does is it allows users to sign up to beta test a particular app in a dedicated Google Group or Google+ community. Users in that community, only, are able to check out the beta or alpha version of the app. This limits the testing pool of users as well as giving users and developers a single place to talk about the app. Whether its bugs, or something else. WhatsApp has now joined in, and has made a beta track available for their app.
WhatsApp isn't using a Google Group or Google+ community for their beta testing - since they are a Facebook company they'd rather drive traffic to Facebook than a competing service like Google+. So that takes out one of the steps needed to sign up. All you need to do is head to their Play Store listing, then opt into become a tester. On that page, there is a link to download WhatsApp if you haven't already downloaded it on your smartphone. If you already have it, then within about an hour or so, you will likely see an update to WhatsApp. There's nothing new in there, yet. It's simply getting you on the beta track.
Now before you decide to opt into the beta, it's important to note that beta's do often times have bugs and other issues. So if you are worried about having bugs and such, you'll want to skip this beta.