Flutter is a development tool kit to develop apps, mobile web and deskop from a single codebase.. meaning you can create apps for Apple and Android phones with a single codebase.. Wow.
Direct quote from their website
I have yet to try this, but its defiantly on my to do list, this would make app development so much simpler and faster if all they say is true.
Here is a video that shows what its all about.
Some great examples of Flutter Apps can be found here with the developer stories.