FlatRedBall projects are standard .NET projects which are saved in a .csproj file. Whenever you work in the FlatRedBall Editor, such as by adding a new Entity or Screen, the FlatRedBall Editor also modifies your .csproj file.
As you work on your game and add additional files to your .csproj file, such as by adding a new .cs file in Visual Studio, the FlatRedBall Editor automatically reloads your project to pick up your changes.