Flutter is an open source SDK from Google that helps you create native apps for modern mobile platforms (currently, Android & iOS) from a single codebase with extensive tooling and widgets support for rapid prototyping.
Compared to other relevant solutions (e.g., React Native, PhoneGap, Cordova), Flutter focuses first on performance by compiling directly to native (ARM) code and by removing the overheads of context-switching to OEM widgets using a UI architecture that works with the GPU to render directly to canvas.
Flutter's layered architecture coupled with its open-source nature, makes it possible for developers to customize everything within the framework, while relying on battle-tested engines for optimized text (from Chrome), graphics (Skia, used in Android) and framework (Dart, used in production at Google) support.
Flutter is currently in alpha release. You can get a quick intro and links to more resources in the talk I gave at #WindyCityDevFest in Chicago in January 2018 or try the codelab for hands-on learning.