Flutter is a toolkit for building user interfaces (UIs), the screens you interact with on an app. It was created by Google to solve a common problem for developers: writing separate code for every platform, like iOS, Android, and the web.

With Flutter, developers can write their code just once and use it to create apps that run smoothly on iPhones, Android phones, web browsers, and even desktop computers. This saves a massive amount of time and effort.

Besides the “write once, run anywhere” benefit, Flutter has a few other key advantages that make it popular.

First, it’s incredibly fast for developers. A feature called Hot Reload lets them see changes to the code reflected in the app almost instantly. Imagine painting a picture and seeing every brushstroke appear on the canvas in real-time. That’s what Hot Reload does for app development, making it easier to experiment and fix bugs.

Second, Flutter apps feel like they truly belong on their device. This is because they are compiled directly into native machine code, the language the device’s processor understands. This results in smooth animations and a responsive feel, without the sluggishness some cross-platform tools can have.

Finally, Flutter gives developers complete control over every pixel on the screen. This allows for highly customized and expressive user interfaces that match a brand's vision perfectly.

Flutter's power comes from its well-designed architecture, which is organized in layers. Think of it like a cake. Each layer has a specific job, and together they create the final product.

1. The Framework Layer: This is the top layer, where developers work. It’s written in a programming language called Dart. The UI of a Flutter app is built by combining small, reusable pieces called widgets. A button is a widget, a line of text is a widget, and even the layout of the whole screen is a widget.

2. The Engine Layer: This is the middle layer, written mostly in C++. It’s the powerhouse that does the heavy lifting, including rendering graphics, handling input and output, and running the Dart code. It uses a high-performance graphics engine called Skia to draw the UI.

3. The Embedder Layer: This is the bottom, platform-specific layer. It acts as the bridge between Flutter and the operating system it’s running on, whether that’s iOS, Android, or Windows. It handles things like the app window, services, and user input.

The entire Flutter framework is written in a language called Dart. Also created by Google, Dart is optimized for building user interfaces. It's designed to be fast, easy to learn, and capable of compiling to that speedy native code mentioned earlier. You don't need to be a Dart expert to start, but it's the language that powers everything you build in Flutter.