#region lets you specify a block of code that you can expand or collapse when using the outlining feature of the Visual Studio Code Editor. For example:
#region MyClass definition
public class MyClass
{
static void Main()
{
}
}
#endregion
A #region block must be terminated with a #endregion directive.
A #region block cannot overlap with a #if block. However, a #region block can be nested in a #if block, and a #if block can be nested in a #region block.
Reference
C# Preprocessor Directives
Concepts
C# Programming Guide
Other Resources
C# Reference