Flutter is a powerful mobile application development framework. It uses dart programming language to create a codebase that can be compiled to multiple platforms such as Android, IOS, Windows, Mac and Linux.
The fact that Flutter is a cross-platform framework is one of the reasons it is most liked, since that means it reduces development costs and team sizes for projects that require multiple platforms. Also its hot-reload feature makes it a very good choice for fast mobile application development.
And since it compiles and runs at near-native speeds, it's more preferred than ohter frameworks like React-Native. Recently Flutter switched their UI rendering engine from the original Skia engine to the new more powerful Impeller engine, vastly improving its speed and performance, specially on IOS platform.