The Play Store menu is adding a new games button among other changes if you're on the latest version of the Play Store app, which is currently version 8.2.56. Worth mentioning though is that after checking this version the changes do not appear, so this may only be showing up for some users, which suggests that Google is slowly rolling this change out to everyone in stages as a server-side change it has to make. If that's the case then there may be a little bit of a wait before you see the changes visually.
As to what the changes are, as mentioned above a new Games button is appearing in the hamburger menu. Currently when you tap the hamburger menu button you'll see options for Apps & Games, My Apps & Games, then a small icon for movies, music and books. In the latest visual change to this menu, the Apps & Games button is gone and the new button on top will be your apps and games which are already installed, and there will be a new Home button that sits just below this, and the Games button will sit below that.
In addition to these changes, the button for movies, music, and books is now gone, and the line separator between that button and the individual buttons for movies & tv, music, books, and newstand is gone as these buttons are all now in the same section as the home and games buttons. Below that you'll still see buttons for your account, the redeem codes section, your wishlist and others just like before, with the line separator in the same place just above the button for your account just like before so there is no change here. For now this seems to be all the changes that have been made, at least visually, which might seem minor and for the most part they are, but Google is known for making little changes like these constantly and they don't always stick around, or sometimes even go unnoticed by a majority of users. While the changes might not be extremely functional changes, it appears Google is looking to simply clean things up a bit and make the menu less cluttered as well as more organized.