What is Dependency Injection in MVC?

In simple words:

Objects do not create other objects on which they rely to do their work. Instead, they get the objects that they need from an outside source.

 

Few in-depth articles here:

https://docs.microsoft.com/en-us/aspnet/core/mvc/controllers/dependency-injection

http://www.dotnetcurry.com/aspnet-mvc/786/dependency-injection-aspnet-mvc-introduction