#endif specifies the end of a conditional directive, which began with the #if directive. For example,
#define DEBUG
// ...
#if DEBUG
Console.WriteLine("Debug version");
#endif
A conditional directive, beginning with a #if directive, must explicitly be terminated with a #endif directive. See #if (C# Reference) for an example of how to use #endif.
Reference
C# Preprocessor Directives
Concepts
C# Programming Guide
Other Resources
C# Reference