Business related projects are moving towards a RESTful web app architecture. So if you want to stay with C# you should learn about the ASP.NET MVC 6 / Web API 2 stuff right now. This is related to the midddle tier and backend layer. Understanding the principles of database design won't hurt either.
It is normal to see job ads like these. But the very core is that you have a good understanding of programming languages in general and it is good to have a sense of problem solving. It is good if you have one language where you're an expert in but it is never wrong to atleast have an intermediate level of experience in various other ones.
In the end I'd say if your more a creative head you should focus more on the front end stuff since it is more related to user friendliness and gives you the opportunity to be creative. (Atleast when the company doesn't have a standard layout or something...).
If your more into problem solving and the technical stuff you should consider going for the middle / backend related tasks.