For all that Minecraft is full of ways to express your creativity, it's always had very few ways to express a greeting. Sure, you can type a message to a friend playing with you but that's such work. Minecraft's Bedrock Edition (that's the Windows 10 one, not the Java launcher) now has emotes you can use to say hello.
Back in my day, you greeted a friend by looking at them and smashing your shift key to crouch over and over or jumping for joy or punching them in the face. Now you can choose from a few new default options like waving and clapping and pointing. I suppose punching your mates is frowned upon, now. Can't have any fun these days, I tell ya what.
There are more emotes to unlock through achievements, but those are the three you'll begin with. To start off, you'll need to open your settings menu and edit your profile to choose which emotes you want equipped. The default key B will open the radial emote menu where you can pick one, like so:
Seems awfully geared towards gamepad users—which would typically be me, except for in Minecraft—but I suppose I'll not throw stones over a radial menu. Clap it up, kids.
Mojang say that the list of available emote achievements will continue to grow over time. The three other emotes you can earn right now are The Hammer, The Pickaxe, and "Diamonds to you!"
Naturally, they're adding some to the Marketplace for you to buy with cash as well. I'd wager we'll wind up with a dab emote somewhere in the Marketplace. Do the kids still dab? I don't know, but maybe Mojang do.
You can peep a couple more gifs over in Mojang's post about the new achievement awards.