Play Store betas offer a simple way to get the newest features in your favorite apps as soon as possible. Google runs a few of them, and today it's adding Play Services and Gboard to the list. Just a few clicks and you can be running the latest goodies (and maybe some bugs) early.
Google bundles up a ton of features and services in Google Play Services, and the wait for new versions to roll out can sometimes be agonizing. That's why Play Services is one of the most popular downloads on APK Mirror. If you want to join the beta, head to the testing link and confirm (make sure you're signed into the right account first). Your devices will download an update to Play Services automatically, which currently means you'll be boosted from v10.2 to v10.5. The exact version number will depend on your device.
You'll continue to get updates to Play Services in the beta channel. If you ever want to stop getting beta updates, return to the testing page and leave the beta. Then either wait for the stable channel to catch up or manually roll back Play Services (you need to disable Android Device Manager in the Administrators menu first). Hopefully you won't need to do that. If Google is making the beta publicly available, it should at least not be a complete mess.
It's the same basic deal with Gboard. Visit the testing link and confirm your interest. The stable version for this one is v6.0 and the beta is v6.1. If you're using devices that don't already have Gboard installed, you'll need to manually install it from the Play Store. This one is easier to roll back if you have problems.