The yarn start command

The function of this command is to start the development server.

Running start on your Create React App project will bring your project from code to your web browser. That is to say, it will take your project's code and compile everything together. From there, it will load a development server with a default starter template. The other nice thing about this is that it will actually pick up any changes you make to any code (assuming you save that code), so you don't have to constantly play the game of make a change, save the file, restart the server, refresh the browser; instead, you will have instant feedback for any of the changes that you make.

Starting off with a completely fresh Create React App project and running start will yield the following: