Chromebooks generally work with a wide array of keyboards, mice, touchpads, gamepads, flash drives, and other accessories, thanks to the Linux kernel at the core of Chrome OS. However, there are still cases where it's not clear if a certain adapter or other accessory will work with Chromebooks, and that's what Google aims to address.
Google today introduced a new 'Works with Chromebook' badge, which you'll start to see soon on some accessories sold in the United States, Canada, and Japan. Products with the badge have been verified to "comply with Chromebook's compatibility standards." It takes the guessing work out of checking if something works with Chrome OS.
Products from several major companies are in the process of being certified, with Logitech, Anker, Belkin, Brydge, Kensington, and others already onboard. Logitech today announced that several of its M-series mice have been certified, with more products in the process. I'm fairly sure every single USB/Bluetooth mouse in existence already works on Chromebooks, but at least now buyers will know for sure.