In a nutshell:
Flutter is a development environment from Google for applications, with which you can develop one and the same program for different operating systems. With Flutter applications are realized like FluffyChat (Matrix client).
Advantages Basically: Practical - Fast development for different operating systems / works quasi everywhere
Advantages compared to Electron: better performance, less attack potential, less memory requirements
Flutter first appeared as an open source project in late 2018 and simplifies the process of app development from Google. Flutter is a development environment of cross-platform applications using the Dart programming language. Primarily, Flutter is used for iOS and Android app development. It can also be used to develop apps for macOS, Windows, Linux and Google Fuchsia.
For apps developed with Flutter, programmers do not have to pay attention to the specifics of the different systems, as they can be developed with just one codebase.
Fast execution speed and short development times are the primary focus of Flutter.
Flutter itself uses the Dart Virtual Machine (Dart VM), as well as the Skia graphics library. The Flutter program was written in C++.
The object-oriented programming is consequently implemented up to the user interface.
The user interface of a Flutter program consists mainly of widgets. These can be nested in each other. Each displayed text or button is a widget with different properties that can be changed. These can influence each other and react to status changes from outside with built-in functions. Furthermore, these can be extended with additional functions as desired.
The Dart programming language has many similarities to other languages in basic development mechanics. Development with Flutter is greatly accelerated because it is co-developed with Dart by Google, so the two build on each other, which speeds up development tremendously.
Other advantages are that Flutter and Dart are open-source and Flutter is free to use.
In addition, extensive documentation and community support are provided.
The fact that only one code base has to be developed for all major operating systems is also an advantage of Flutter, as are the prefabricated UI elements, which are fixed components of the software.
Source: https://www.twigbit.com/glossar/flutter (external)
More info on the net: