In short: Amazon has redesigned the Alexa app for both Android and iOS, to make it easier to control your smart home products, as well as interacting with Alexa. The app makes it easier to create groups and add lights or other smart home products to those groups. It also makes it easier, visually at least, to turn on or off those products either as a group or individually. You can also view all of your Alexa-compatible devices on your account pretty easily on the Smart Home devices screen, and see what you want to turn on or off. Everything is basically in the same place, but the app is now faster, it also makes it easier to do different things - including searching for new Alexa skills and so much more.
Background: The Amazon Alexa app is the app that you use to set up an Echo or connect a smart home product to your Alexa device. It can also be used for messaging and making calls through Alexa, as well as finding new skills to add to Alexa. Basically, it's a very important part of Alexa, and something that was desperately in need of an update. Now Amazon has redesigned the app, making it much easier to use, and it is also visually better to use as well.
Impact: As mentioned already, the app update is rolling out now to both Android and iOS devices. So if you don't have the update right now, it should be available pretty soon. This is a staged rollout, as per usual for Amazon - for example, we have the update on a Galaxy Note 9 but not a Pixel 2 XL, using the same Google Play account. So if the update is not yet available for you, hang tight and it should be available pretty soon.