Asp.Net Core is a toolset for creating cross-platform web applications using .Net Core. It allows developers to write more compact code and supports self-hosting with the Kestrel web server. The text provides a step-by-step guide on how to create a simple ASP.Net Core application from scratch without any code generators or IDEs, using only terminal commands and a favorite text editor. It also covers adding MVC support, handling different types of requests, working with JSON data, and utilizing dependency injection in the application.