My understanding from small MVC applications is that you have the front end, which deals with HTML, JS, jQuery, etc, and you have the back end, which consists of your controllers and models. 

However, when I talk to developers from large companies, they often mention having a frontend tier and a backend tier. So sometimes, I might hear that they have a frontend with C# and a backend with Java. Why would any company want a backend and frontend in different languages? Does this help the large website scale better?

When people say that their frontend is built in C#, does this mean that they are using a framework for the frontend (like .NET) and an additional framework on the backend (such as Spring)? Or does it mean something entirely different?

"Front-end" and "Back-end" can be nebulous terms, particularly in enterprise applications. "Front-end" can mean the UI, or it can be the entire application. "Back-end" can be used to mean the internals, or it could be the database or external services that are consumed. What the terms mean often depend entirely who are you talking to. So did you maybe ask "hey, what do you mean by that?"

When you get into large enterprise development, you are going to have lots and lots of teams writing lots and lots of code. These teams will be developing in different languages, using different paradigms, from different locations. Some of this code will need to work together and much of it will not. 

I work for a large bank. My team develops our application in C#. All of it. But we consume web services that are largely written in Java, and those services talk to other services that talk to other services that get the account data to and from the appropriate data stores, and who knows what languages are used with those.

Short version: People use the tools that get the job done.
