https://docs.microsoft.com/en-us/dotnet/api/system.web.ihttphandler?view=netframework-4.8
Defines the contract that ASP.NET implements to synchronously process HTTP Web requests using custom HTTP handlers.
Remarks
You can write custom HTTP handlers to process specific, predefined types of HTTP requests in any Common Language Specification (CLS) compliant language.
Executable code defined in the HttpHandler classes, rather than conventional ASP or ASP.NET Web pages, responds to these specific requests.
HTTP handlers give you a means of interacting with the low-level request and response services of the IIS Web server and provide functionality much like ISAPI extensions but with a simpler programming model.
If your handler will access session state values, it must implement the IRequiresSessionState interface (a marker interface with no methods).