Hi mukesh1,

HTTP handler is the process that runs in response to a request that is made to an ASP.NET Web application. The most common handler is an ASP.NET page handler that processes .aspx files. When users request an .aspx file, the request is processed by the page handler.

For more details refer below link.

HTTP Handlers

To use refer below article.

ASP.Net: Call (Execute) Generic HTTP Handler from Code Behind using C# and VB.Net