The "System.Web.Http.dll" is a file that's essential for building and running web applications using the Microsoft ASP.NET MVC framework. ASP.NET MVC is a programming framework for creating web applications, and the DLL (Dynamic Link Library) in question provides many of the necessary tools and libraries to help achieve this. Essentially, it's a collection of pre-written code that developers can call upon when building their projects. Inside this DLL are many classes and methods that handle various aspects of web development, such as routing, model binding, and request handling, among others. These functionalities are pivotal for building robust, scalable, and maintainable web applications. When you visit a website built on the ASP.NET MVC framework, this DLL works behind the scenes, taking care of tasks like processing your requests and generating the appropriate responses, all while ensuring smooth interaction between the web server and the application's backend logic.

So, for the average user, while you might never interact with this DLL directly, it's an integral part of many web applications you use. For developers, it's a vital component that enables them to create complex and feature-rich web apps without having to build everything from scratch. It helps streamline the development process, making it quicker and more efficient.