ASP.NET is a web developement framework designed to help website developers build applications faster. It is popular because it's relatively easy to create fast, simple websites that can scale up to millions of visitors.
Using a development framework allows you to speed up and reduce development costs, but it does mean you need to hire a developer with knowledge of working with the framework, in this case, ASP.
Freelance ASP.NET Developers
If you need to developer to help with your web application, an experienced ASP.NET developer will be able help you with:
Using MVC architecture
Improving application security
Creating new applications or improving existing ones